Fix Scoverage report generation in Scala 3 (#3936)

Fixes #3857. We were missing
one of the necessary flags, which is documented in
https://dotty.epfl.ch/docs/internals/coverage.html

Manually tested, reproduced the problem and verified the fix. I'm not
sure why the existing unit tests didn't catch it, must be something to
do with how the sandbox is set up for unit tests vs in real usage. I'm
not sure if it's worth introducing an integration test just for this so
just leaving it at manual testing for now

contrib/scoverage/src/mill/contrib/scoverage/ScoverageModule.scala

@@ -174,7 +174,10 @@ trait ScoverageModule extends ScalaModule { outer: ScalaModule =>
       Task {
         val extras =
           if (isScala3()) {
-            Seq(s"-coverage-out:${data().path.toIO.getPath()}")
+            Seq(
+              s"-coverage-out:${data().path.toIO.getPath()}",
+              s"-sourceroot:${T.workspace}"
+            )
           } else {
             Seq(
               s"-P:scoverage:dataDir:${data().path.toIO.getPath()}",