ci(auto-sync): pin gh CLI to GITHUB_REPOSITORY via GH_REPO

The conflict-case path runs 'git checkout -B ${PR_BRANCH} upstream/main'
which sets the new branch's upstream-tracking to the upstream remote
(router-for-me/CLIProxyAPI). The gh CLI then infers the current repo
from that tracking and targets upstream — leading to:

  HTTP 403: Resource not accessible by integration
  (https://api.github.com/repos/router-for-me/CLIProxyAPI/labels)
  could not add label: 'auto-sync' not found

Setting GH_REPO at the step level pins every gh invocation to this
repository regardless of git remote state.

Author: conversun

.github/workflows/auto-sync-upstream.yml

@@ -132,6 +132,7 @@ jobs:
         if: steps.check.outputs.behind != '0' && (steps.merge.outputs.clean != 'true' || steps.build.outputs.ok != 'true')
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_REPO: ${{ github.repository }}
         run: |
           set -e