Don't use ProxyTerminal in background job

If we use the ProxyTerminal in the background jobs, the logs
would be spread across different terminals, switching from active
client to active client. We want the logs to stick
to the client that started the job.

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

@@ -438,6 +438,7 @@ object Terminal {
     override def toString: String = s"ProxyTerminal(current = $t)"
   }
   private[sbt] def get: Terminal = ProxyTerminal
+  private[sbt] def current: Terminal = activeTerminal.get
 
   private[sbt] def withIn[T](in: InputStream)(f: => T): T = {
     val original = inputStream.get

main/src/main/scala/sbt/internal/LogManager.scala

@@ -149,7 +149,7 @@ object LogManager {
         task: ScopedKey[_],
         context: LoggerContext
     ): ManagedLogger = {
-      val console = screen(task, state)
+      val console = ConsoleAppender("bg-" + ConsoleAppender.generateName(), ITerminal.current)
       LogManager.backgroundLog(data, state, task, console, relay(()), context)
     }
   }