Clean scala2-library on scala3-bootstrapped/clean (#19451)

Follow-up of #19447

Author: odersky

project/Build.scala

@@ -2117,6 +2117,9 @@ object Build {
       dependsOn(tastyCore).
       dependsOn(dottyCompiler).
       dependsOn(dottyLibrary).
+      bootstrappedSettings(
+        addCommandAlias("clean", ";scala3-bootstrapped/clean;scala2-library-bootstrapped/clean;scala2-library-cc/clean"),
+      ).
       nonBootstrappedSettings(
         addCommandAlias("run", "scala3-compiler/run"),
         // Clean everything by default