Compile the community-build with the bootstrapped compiler

Previously we compiled it with the reference compiler. Both work, but
using the bootstrapped compiler means we can catch potential
regressions before we update the reference compiler to a new version.

Author: smarter

project/Build.scala

@@ -977,12 +977,9 @@ object Build {
   val updateCommunityBuild = taskKey[Unit]("Updates the community build.")
 
   lazy val `community-build` = project.in(file("community-build")).
-    settings(commonSettings).
+    dependsOn(dottyLibrary(Bootstrapped)).
+    settings(commonBootstrappedSettings).
     settings(
-      scalaVersion := referenceVersion,
-      // To be removed once we stop cross-compiling with Scala 2
-      crossScalaVersions := Seq(referenceVersion, scalacVersion),
-
       prepareCommunityBuild := {
         (publishLocal in `dotty-sbt-bridge`).value
         (publishLocal in `dotty-interfaces`).value