fix: pass github-token to primary checkout in stable-sync action

The main repo checkout step didn't forward the elevated github-token
input to actions/checkout, so it silently fell back to the caller's
default GITHUB_TOKEN for persisted git credentials. When a consuming
workflow scopes its job-level permissions down to just id-token: write
(as metamask-mobile's stable-branch-sync.yml does after migrating to
OIDC token exchange), that default token has no contents:write, and
the branch push later in this action fails with a 403:

  remote: Permission to MetaMask/metamask-mobile.git denied to github-actions[bot].
  fatal: unable to access '...': The requested URL returned error: 403

Since the push fails, the subsequent "Create Pull Request" step is
skipped, so no stable -> main sync PR ever gets opened.

This mirrors the sibling release-branch-sync action, which already
passes token: ${{ inputs.github-token }} on its primary checkout.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: tommasini

.github/actions/stable-sync/action.yml

@@ -31,6 +31,7 @@ runs:
     - uses: actions/checkout@v6
       with:
         fetch-depth: 0
+        token: ${{ inputs.github-token }}
 
     - name: Checkout GitHub tools repository
       uses: actions/checkout@v6