Change the token used for mirroring. (#4646)

This is required to be able to close the repo during the default branch
migration.

Bug: flutter/flutter#90476

Author: godofredoc

.github/workflows/mirror.yml

@@ -20,6 +20,6 @@ jobs:
         id: mirror
         uses: google/mirror-branch-action@c6b07e441a7ffc5ae15860c1d0a8107a3a151db8
         with:
-          github-token: ${{ secrets.FLUTTERGITHUBBOT_TOKEN }}
+          github-token: ${{ secrets.FLUTTERMIRRORINGBOT_TOKEN }}
           source: 'master'
           dest: 'main'