Handle relative path in dottyStyleMessage.

Author: PapyChacal

libs/javalib/worker/src/mill/javalib/zinc/TransformingReporter.scala

@@ -82,7 +82,12 @@ private object TransformingReporter {
     InterfaceUtil.jo2o(pos.sourcePath()) match {
       case None => message
       case Some(path) =>
-        val absPath = os.Path(path)
+        val absPath = os.FilePath(path) match {
+          case abs: os.Path => abs
+          // Assume relative paths are relative to the current workspaceRoot
+          case rel: os.RelPath => workspaceRoot / rel
+          case sub: os.SubPath => workspaceRoot / sub
+        }
         // Render paths within the current workspaceRoot as relative paths to cut down on verbosity
         val displayPath =
           if absPath.startsWith(workspaceRoot) then absPath.subRelativeTo(workspaceRoot).toString