Change GitHub token to use COPILOT_PAT

Author: aarongustafson

.github/workflows/assign-to-copilot.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Add Copilot mention comment
         uses: actions/github-script@v7
         with:
-          github-token: ${{ secrets.GITHUB_TOKEN }}
+          github-token: ${{ secrets.COPILOT_PAT }}
           script: |
             const issueNumber = ${{ inputs.issue_number }};