Allow upstream delete continue on failure

.github/workflows/contributors.yml

@@ -11,6 +11,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Delete upstream contributor branch
+        # Allow continue on failure to account for when the
+        # upstream branch is deleted or does not exist.
+        continue-on-error: true
         run: git push origin --delete update-contributors
       - name: Create up-to-date contributors branch
         run: git checkout -B update-contributors