[2.x] fix: Fixes managedScalaInstance false support (#9121)

**Problem**
When managedScalaInstance is set to false, we stopped creating
ScalaInstance, but the documentation says the user can
pull in their own dependencies.

**Solution**
Attempt to construct a ScalaInstance from update report
even when managedScalaInstance is set to false.

main/src/main/scala/sbt/Defaults.scala

@@ -769,9 +769,8 @@ object Defaults extends BuildCommon with DefExtra {
       scalaCompilerBridgeBin := Def
         .ifS(Def.task {
           val sv = scalaVersion.value
-          val managed = managedScalaInstance.value
           val hasSbtBridge = ScalaArtifacts.isScala3(sv) || ScalaArtifacts.hasScala2SbtBridge(sv)
-          hasSbtBridge && managed
+          hasSbtBridge
         })(Compiler.compilerBridgeFromUpdate)(Def.task(Vector.empty))
         .value,
       scalaCompilerBridgeJars := (Def.taskDyn {

main/src/main/scala/sbt/internal/Compiler.scala

@@ -64,12 +64,14 @@ object Compiler:
       val sh = Keys.scalaHome.value
       val app = Keys.appConfiguration.value
       val managed = Keys.managedScalaInstance.value
-      sh match
-        case Some(h) => scalaInstanceConfigFromHome(h)
-        case _ =>
-          val scalaProvider = app.provider.scalaProvider
-          if !managed then emptyScalaInstanceConfig
-          else scalaInstanceConfigFromUpdate(extraToolConf)
+      val configs = Keys.ivyConfigurations.value
+      (sh, extraToolConf) match
+        case (Some(h), _) => scalaInstanceConfigFromHome(h)
+        case _ if !managed =>
+          val extra = extraToolConf.getOrElse(Configurations.ScalaTool)
+          if configs.contains(extra) then scalaInstanceConfigFromUpdate(extraToolConf)
+          else emptyScalaInstanceConfig
+        case _ => scalaInstanceConfigFromUpdate(extraToolConf)
     }
 
   /**

sbt-app/src/sbt-test/project/scala-instance/A.scala

@@ -0,0 +1,5 @@
+package example
+
+class A {
+  val x = 1
+}

sbt-app/src/sbt-test/project/scala-instance/build.sbt

@@ -0,0 +1,19 @@
+import Configurations.{ ScalaTool, ScalaDocTool, ZincTool }
+
+@transient
+lazy val check = taskKey[Unit]("")
+lazy val scala213 = "2.13.16"
+scalaVersion := scala213
+autoScalaLibrary := false
+managedScalaInstance := false
+ivyConfigurations ++= List(ScalaTool, ScalaDocTool, ZincTool)
+libraryDependencies ++= Seq(
+  "org.scala-lang" % "scala-library" % scala213,
+  "org.scala-lang" % "scala-compiler" % scala213 % ScalaTool,
+  "org.scala-lang" % "scala-compiler" % scala213 % ScalaDocTool,
+  "org.scala-lang" % "scala2-sbt-bridge" % scala213 % ZincTool,
+)
+check := {
+  val si = scalaInstance.value
+  assert(si.version == scala213, s"'${si.version}' was not '$scala213'")
+}

sbt-app/src/sbt-test/project/scala-instance/test

@@ -0,0 +1,3 @@
+> compile
+> doc
+> check