[snapshot release]: Push the snapshot to a snapshot branch (#6896)

* [snapshot release]: Push the snapshot to a snapshot branch

The external bot can track the particular branch to sync to the latest
snapshot.

This fixes https://github.com/google/iree/issues/5143

.github/workflows/validate_and_publish_release.yml

diff --git a/.github/workflows/validate_and_publish_release.yml b/.github/workflows/validate_and_publish_release.yml
index 5ae68f7..d9fb8e5 100644
--- a/.github/workflows/validate_and_publish_release.yml
+++ b/.github/workflows/validate_and_publish_release.yml
@@ -24,3 +24,14 @@
           GITHUB_TOKEN: ${{ secrets.WRITE_ACCESS_TOKEN }}
         with:
           release_id: ${{ github.event.inputs.release_id }}
+
+      - name: Checking out repository
+        uses: actions/checkout@v2
+        with:
+          token: ${{ secrets.WRITE_ACCESS_TOKEN }}
+
+      - name: Updating latest-snapshot branch
+        uses: ad-m/github-push-action@v0.6.0
+        with:
+          github_token: ${{ secrets.WRITE_ACCESS_TOKEN }}
+          branch: latest-snapshot