Make sure the community build runs with the correct ivy cache

This is necessary for the community build to work on the CI now that we
run the community build with fork in Test set to true.

Author: smarter

project/Build.scala

@@ -1101,7 +1101,18 @@ object Build {
         "--include-categories=dotty.communitybuild.TestCategory",
       ),
       (Test / testOnly) := ((Test / testOnly) dependsOn prepareCommunityBuild).evaluated,
-      (Test / test    ) := ((Test / test    ) dependsOn prepareCommunityBuild).value
+      (Test / test    ) := ((Test / test    ) dependsOn prepareCommunityBuild).value,
+      javaOptions ++= {
+        // Propagate the ivy cache directory setting to the tests, which will
+        // then propagate it further to the sbt instances they will spawn.
+        val sbtProps = Option(System.getProperty("sbt.ivy.home")) match {
+          case Some(ivyHome) =>
+            Seq(s"-Dsbt.ivy.home=$ivyHome")
+          case _ =>
+            Seq()
+        }
+        sbtProps
+      }
     )
 
   lazy val publishSettings = Seq(