chore: add SSH_PRIVATE_KEY for auth (#12)

Author: RoyalOughtness

.github/workflows/upstream-sync.yml

@@ -41,6 +41,10 @@ jobs:
           git_user_signingkey: true
           git_commit_gpgsign: true
           git_push_gpgsign: true
+          
+      - uses: webfactory/ssh-agent@a6f90b1f127823b31d4d4a8d96047790581349bd # v0.9.1
+        with:
+            ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}
 
       - name: Pull tags from upstream and rebase
         shell: bash