Enable -Ycheck:all for scala2-library-bootstrapped

Author: liufengyun

project/Build.scala

@@ -1045,7 +1045,10 @@ object Build {
     dependsOn(dottyCompiler(Bootstrapped) % "provided; compile->runtime; test->test").
     settings(commonBootstrappedSettings).
     settings(scala2LibraryBootstrappedSettings).
-    settings(moduleName := "scala2-library")
+    settings(moduleName := "scala2-library").
+    settings(
+      scalacOptions += "-Ycheck:all",
+    )
 
   /** Scala 2 library compiled by dotty using the latest published sources of the library.
    *