Change the token used for mirroring. (#4646)

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

Bug: https://github.com/flutter/flutter/issues/90476
diff --git a/.github/workflows/mirror.yml b/.github/workflows/mirror.yml
index 47af473..c9780f7 100644
--- a/.github/workflows/mirror.yml
+++ b/.github/workflows/mirror.yml
@@ -20,6 +20,6 @@
         id: mirror
         uses: google/mirror-branch-action@c6b07e441a7ffc5ae15860c1d0a8107a3a151db8
         with:
-          github-token: ${{ secrets.FLUTTERGITHUBBOT_TOKEN }}
+          github-token: ${{ secrets.FLUTTERMIRRORINGBOT_TOKEN }}
           source: 'master'
           dest: 'main'