Fix casting error during initialization

While running scripted, you see

```
ERROR StatusLogger Unable to create custom ContextSelector. Falling back to default.
 java.lang.ClassCastException: Cannot cast org.apache.logging.log4j.core.async.AsyncLoggerContextSelector to org.apache.logging.log4j.core.selector.ContextSelector
	at java.lang.Class.cast(Class.java:3369)
	at org.apache.logging.log4j.util.LoaderUtil.newCheckedInstanceOf(LoaderUtil.java:201)
	at org.apache.logging.log4j.util.LoaderUtil.newCheckedInstanceOfProperty(LoaderUtil.java:226)
	at org.apache.logging.log4j.core.impl.Log4jContextFactory.createContextSelector(Log4jContextFactory.java:97)
	at org.apache.logging.log4j.core.impl.Log4jContextFactory.<init>(Log4jContextFactory.java:58)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at java.lang.Class.newInstance(Class.java:442)
	at org.apache.logging.log4j.LogManager.<clinit>(LogManager.java:94)
	at org.apache.logging.log4j.spi.ThreadContextMapFactory.createThreadContextMap(ThreadContextMapFactory.java:73)
	at org.apache.logging.log4j.ThreadContext.init(ThreadContext.java:223)
	at org.apache.logging.log4j.ThreadContext.<clinit>(ThreadContext.java:202)
	at org.apache.logging.log4j.core.impl.ContextDataInjectorFactory.createDefaultInjector(ContextDataInjectorFactory.java:83)
	at org.apache.logging.log4j.core.impl.ContextDataInjectorFactory.createInjector(ContextDataInjectorFactory.java:67)
	at org.apache.logging.log4j.core.lookup.ContextMapLookup.<init>(ContextMapLookup.java:34)
	at org.apache.logging.log4j.core.lookup.Interpolator.<init>(Interpolator.java:117)
	at org.apache.logging.log4j.core.config.AbstractConfiguration.<init>(AbstractConfiguration.java:125)
	at org.apache.logging.log4j.core.config.DefaultConfiguration.<init>(DefaultConfiguration.java:46)
	at org.apache.logging.log4j.core.layout.PatternLayout$Builder.build(PatternLayout.java:650)
	at org.apache.logging.log4j.core.layout.PatternLayout.createDefaultLayout(PatternLayout.java:487)
	at sbt.internal.util.ConsoleAppender.<init>(ConsoleAppender.scala:245)
```

This aims to workaround the casting error during PatternLayout.createDefaultLayout()
that was originally used for ConsoleAppender.
The stacktrace shows it's having issue initializing default DefaultConfiguration.
Since we currently do not use Layout inside ConsoleAppender, the actual pattern is not relevant.

internal/util-logging/src/main/scala/sbt/internal/util/ConsoleAppender.scala

@@ -8,7 +8,6 @@ import org.apache.logging.log4j.{ Level => XLevel }
 import org.apache.logging.log4j.message.{ Message, ObjectMessage, ReusableObjectMessage }
 import org.apache.logging.log4j.core.{ LogEvent => XLogEvent }
 import org.apache.logging.log4j.core.appender.AbstractAppender
-import org.apache.logging.log4j.core.layout.PatternLayout
 
 import ConsoleAppender._
 
@@ -242,7 +241,7 @@ class ConsoleAppender private[ConsoleAppender] (
   val ansiCodesSupported: Boolean,
   val useColor: Boolean,
   val suppressedMessage: SuppressedTraceContext => Option[String]
-) extends AbstractAppender(name, null, PatternLayout.createDefaultLayout(), true) {
+) extends AbstractAppender(name, null, LogExchange.dummyLayout, true) {
   import scala.Console.{ BLUE, GREEN, RED, RESET, YELLOW }
 
   def append(event: XLogEvent): Unit =

internal/util-logging/src/main/scala/sbt/util/LogExchange.scala

@@ -5,6 +5,7 @@ import org.apache.logging.log4j.{ LogManager => XLogManager, Level => XLevel }
 import org.apache.logging.log4j.core._
 import org.apache.logging.log4j.core.appender.AsyncAppender
 import org.apache.logging.log4j.core.config.{ AppenderRef, LoggerConfig }
+import org.apache.logging.log4j.core.layout.PatternLayout
 import scala.collection.JavaConverters._
 import scala.collection.concurrent
 import sjsonnew.JsonFormat
@@ -47,6 +48,22 @@ sealed abstract class LogExchange {
     val config = ctx.getConfiguration
     config.getLoggerConfig(loggerName)
   }
+
+  // This is a dummy layout to avoid casting error during PatternLayout.createDefaultLayout()
+  // that was originally used for ConsoleAppender.
+  // The stacktrace shows it's having issue initializing default DefaultConfiguration.
+  // Since we currently do not use Layout inside ConsoleAppender, the actual pattern is not relevant.
+  private[sbt] lazy val dummyLayout: PatternLayout = {
+    val _ = context
+    val ctx = XLogManager.getContext(false) match { case x: LoggerContext => x }
+    val config = ctx.getConfiguration
+    val lo = PatternLayout.newBuilder
+      .withConfiguration(config)
+      .withPattern(PatternLayout.SIMPLE_CONVERSION_PATTERN)
+      .build
+    lo
+  }
+
   def jsonCodec[A](tag: String): Option[JsonFormat[A]] =
     jsonCodecs.get(tag) map { _.asInstanceOf[JsonFormat[A]] }
   def hasJsonCodec(tag: String): Boolean =
@@ -63,9 +80,6 @@ sealed abstract class LogExchange {
   private[sbt] def buildAsyncStdout: AsyncAppender = {
     val ctx = XLogManager.getContext(false) match { case x: LoggerContext => x }
     val config = ctx.getConfiguration
-    // val layout = PatternLayout.newBuilder
-    //   .withPattern(PatternLayout.SIMPLE_CONVERSION_PATTERN)
-    //   .build
     val appender = ConsoleAppender("Stdout")
     // CustomConsoleAppenderImpl.createAppender("Stdout", layout, null, null)
     appender.start