fix(ci): replace SWIFTLM_PR_TOKEN with GITHUB_TOKEN in upstream sync workflow (#7)

The SWIFTLM_PR_TOKEN PAT (belonging to solderzzc) has lost write access
to SharpAI/mlx-swift, causing the upstream sync job to fail with HTTP 403.

The repo already has default_workflow_permissions=write and
can_approve_pull_request_reviews=true, so GITHUB_TOKEN is sufficient
for both force-pushing the sync branch and creating the PR.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: solderzzc

.github/workflows/upstream-sync.yml

@@ -17,7 +17,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
-          token: ${{ secrets.SWIFTLM_PR_TOKEN }}
+          token: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Configure Git
         run: |
@@ -38,7 +38,7 @@ jobs:
 
       - name: Create Pull Request to Main
         env:
-          GITHUB_TOKEN: ${{ secrets.SWIFTLM_PR_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           gh pr create --base main --head sync/upstream-latest \
             --title "🔄 Auto-Sync: Apple Upstream Repository" \