Use custom classloader for scripted tests

We had previously used reflection to load the bridge class, but
continued using sbt's default classloader. This was problematic because
the metabuild could have a different classpath from that required by the
scripted tests.

Bonus: scalafmt

Fixes: #4514

project/Scripted.scala

@@ -17,8 +17,10 @@ object LocalScriptedPlugin extends AutoPlugin {
 trait ScriptedKeys {
   val publishAll = taskKey[Unit]("")
   val publishLocalBinAll = taskKey[Unit]("")
-  val scriptedUnpublished = inputKey[Unit]("Execute scripted without publishing sbt first. " +
-        "Saves you some time when only your test has changed")
+  val scriptedUnpublished = inputKey[Unit](
+    "Execute scripted without publishing sbt first. " +
+      "Saves you some time when only your test has changed"
+  )
   val scriptedSource = settingKey[File]("")
   val scriptedPrescripted = taskKey[File => Unit]("")
 }
@@ -116,7 +118,11 @@ object Scripted {
       ): Unit
     }
 
-    val bridge = bridgeClass.getDeclaredConstructor().newInstance().asInstanceOf[SbtScriptedRunner]
+    val initLoader = Thread.currentThread.getContextClassLoader
+    try {
+      Thread.currentThread.setContextClassLoader(loader)
+      val bridge =
+        bridgeClass.getDeclaredConstructor().newInstance().asInstanceOf[SbtScriptedRunner]
 
       try {
         // Using java.util.List to encode File => Unit.
@@ -135,5 +141,8 @@ object Scripted {
           callback,
         )
       } catch { case ite: InvocationTargetException => throw ite.getCause }
+    } finally {
+      Thread.currentThread.setContextClassLoader(initLoader)
+    }
   }
 }