Test sbt-dotty on sbt 1.5

Author: adpi2

.github/workflows/ci.yaml

@@ -282,9 +282,12 @@ jobs:
       - name: Add SBT proxy repositories
         run: cp -vf .github/workflows/repositories /root/.sbt/ ; true
 
-      - name: Test
+      - name: Test sbt 1.4.x
         run: ./project/scripts/sbt "sbt-dotty/scripted; sbt-community-build/scripted"
 
+      - name: Test sbt 1.5.x
+        run: ./project/scripts/sbt "set \`sbt-dotty\`/scriptedSbt := \"1.5.0-RC1\"; sbt-dotty/scripted sbt-dotty/*; sbt-community-build/scripted"
+
   test_java8:
     runs-on: [self-hosted, Linux]
     container:

project/Build.scala

@@ -1191,6 +1191,9 @@ object Build {
         baseDirectory.value / "../language-server/src/dotty/tools/languageserver/config",
       sbtTestDirectory := baseDirectory.value / "sbt-test",
 
+      // build sbt-dotty with sbt 1.4.9 to make sure it is compatible with sbt 1.4.x
+      pluginCrossBuild / sbtVersion := "1.4.9",
+
       // The batch mode accidentally became the default with no way to disable
       // it in sbt 1.4 (https://github.com/sbt/sbt/issues/5913#issuecomment-716003195).
       // We enable it explicitly here to make it clear that we're using it.