[snapshot] Fix checkout fetch depth to update the latest-snapshot branch (#6945)

* [snapshot] Fix checkout fetch depth to update the latest-snapshot branch

Fix https://github.com/google/iree/runs/3491586526?check_suite_focus=true
Use fetch-depth: 0 to get the full history instead.
Also update the remote branch name.
diff --git a/.github/workflows/validate_and_publish_release.yml b/.github/workflows/validate_and_publish_release.yml
index a57aaa3..2124c8d 100644
--- a/.github/workflows/validate_and_publish_release.yml
+++ b/.github/workflows/validate_and_publish_release.yml
@@ -29,9 +29,12 @@
         uses: actions/checkout@v2
         with:
           token: ${{ secrets.WRITE_ACCESS_TOKEN }}
+          # Get all history. Otherwise the latest-snapshot branch can't be
+          # fast-forwarded.
+          fetch-depth: 0
 
       - name: Updating latest-snapshot branch
         uses: ad-m/github-push-action@v0.6.0
         with:
           github_token: ${{ secrets.WRITE_ACCESS_TOKEN }}
-          branch: refs/heads/latest-snapshot
+          branch: latest-snapshot