Use a fixed 2.13.13 version for sbt scripted tests using Scala.js

Author: WojciechMazur

project/Build.scala

@@ -1708,6 +1708,8 @@ object Build {
         "-Dplugin.version=" + version.value,
         "-Dplugin.scalaVersion=" + dottyVersion,
         "-Dplugin.scala2Version=" + stdlibVersion(Bootstrapped),
+        // The last version of Scala 2 that's cross-published for Scala.js 1.12 (version used by LTS)
+        "-Dplugin.scala2ForJSVersion=2.13.13",
         "-Dplugin.scalaJSVersion=" + scalaJSVersion,
         "-Dsbt.boot.directory=" + ((ThisBuild / baseDirectory).value / ".sbt-scripted").getAbsolutePath // Workaround sbt/sbt#3469
       ),

sbt-test/scala2-compat/erasure-scalajs/build.sbt

@@ -1,7 +1,7 @@
 lazy val scala2Lib = project.in(file("scala2Lib"))
   .enablePlugins(ScalaJSPlugin)
   .settings(
-    scalaVersion := sys.props("plugin.scala2Version")
+    scalaVersion := sys.props("plugin.scala2ForJSVersion")
   )
 
 lazy val dottyApp = project.in(file("dottyApp"))