add -sourceroot flag when generating semanticdb with scala 3 (#3595)

while developing Scala 3 port, when using metals as the BSP server then
semanticdb files are generated in the same folder as the source file.

With this change, and setting the bootstrapped mill version for BSP
server, metals no longer puts semanticdb in the same directory, but
instead in `out/../semanticdbData.dest/classes/META-INF/semanticdb/...`

scalalib/src/mill/scalalib/ScalaModule.scala

@@ -629,7 +629,7 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase { outer =>
       allScalacOptions() ++
         semanticDbEnablePluginScalacOptions() ++ {
           if (ZincWorkerUtil.isScala3(sv)) {
-            Seq("-Xsemanticdb")
+            Seq("-Xsemanticdb", s"-sourceroot:${T.workspace}")
           } else {
             Seq(
               "-Yrangepos",