Stub out mill version for selective testing (#4126)

Needs to happen separately from
#4118 since I need it on both
the PR branch and on the main target branch for the selective testing to
be effective

build.mill

@@ -256,7 +256,11 @@ object Deps {
   }
 }
 
-def millVersion: T[String] = Task { VcsVersion.vcsState().format() }
+def millVersion: T[String] = Task {
+  if (Task.env.contains("MILL_STABLE_VERSION")) VcsVersion.calcVcsState(Task.log).format()
+  else "DEV"
+}
+
 
 def millLastTag: T[String] = Task {
   VcsVersion.vcsState().lastTag.getOrElse(