[libc++] Run macOS jobs after the Stage 2 CI (llvm#141816)

Our CI is currently having major difficulties, which causes the Stage 3
CI to basically never succeed. As a result, our macOS CI jobs have not
been running recently. This patch gates the macOS CI jobs on Stage 2
instead of Stage 3 so that they actually run sometimes.

Author: ldionne

.github/workflows/libcxx-build-and-test.yaml

@@ -184,7 +184,7 @@ jobs:
             **/crash_diagnostics/*
 
   macos:
-    needs: [ stage3 ]
+    needs: [ stage2 ]
     strategy:
       fail-fast: false
       matrix: