github actions: Use correct credentials for pushing

.github/workflows/main-branch-sync.yml

@@ -12,6 +12,10 @@ jobs:
       - name: Checkout Code
         uses: actions/checkout@v2
         with:
+          # persist-credentials: false allows us to use our own credentials for
+          # pushing to the repository.  Otherwise, the default github actions token
+          # is used.
+          persist-credentials: false
           fetch-depth: 0
 
       - name: Update branch