Fix artifactName for os-lib-watch (#243)

Probably during the update of Mill to 0.11 the `artifactName` of
`os-lib-watch` was not set correctly, changing it to `os-watch`.

build.sc

@@ -141,6 +141,7 @@ object os extends Module {
   object watch extends Module {
     object jvm extends Cross[WatchJvmModule](scalaVersions)
     trait WatchJvmModule extends OsLibModule {
+      def artifactName = "os-lib-watch"
       def moduleDeps = super.moduleDeps ++ Seq(os.jvm())
       def ivyDeps = Agg(Deps.jna)
       object test extends ScalaTests with OsLibTestModule {