Make SBT clean clean scala2-library-bootstrapped and scala2-library-cc

[Cherry-picked c5c5f92]

Author: nicolasstucki

project/Build.scala

@@ -1905,7 +1905,7 @@ object Build {
       nonBootstrappedSettings(
         addCommandAlias("run", "scala3-compiler/run"),
         // Clean everything by default
-        addCommandAlias("clean", ";scala3/clean;scala3-bootstrapped/clean"),
+        addCommandAlias("clean", ";scala3/clean;scala3-bootstrapped/clean;stdlib-bootstrapped/clean"),
         // `publishLocal` on the non-bootstrapped compiler does not produce a
         // working distribution (it can't in general, since there's no guarantee
         // that the non-bootstrapped library is compatible with the