[snapshot release] Fix push command for the latest-snapshot branch (#6932)

Fix https://github.com/google/iree/runs/3480016077?check_suite_focus=true
diff --git a/.github/workflows/validate_and_publish_release.yml b/.github/workflows/validate_and_publish_release.yml
index d9fb8e5..a57aaa3 100644
--- a/.github/workflows/validate_and_publish_release.yml
+++ b/.github/workflows/validate_and_publish_release.yml
@@ -34,4 +34,4 @@
         uses: ad-m/github-push-action@v0.6.0
         with:
           github_token: ${{ secrets.WRITE_ACCESS_TOKEN }}
-          branch: latest-snapshot
+          branch: refs/heads/latest-snapshot