Reverse the order of mirroring. (#29753)

This is because now the default branch is main.

Bug: flutter/flutter#90476

Author: godofredoc

.github/workflows/mirror.yml

@@ -6,7 +6,7 @@
 on:
   push:
     branches:
-      - 'master'
+      - 'main'
 
 jobs:
   mirror_job:
@@ -21,5 +21,5 @@ jobs:
         uses: google/mirror-branch-action@c6b07e441a7ffc5ae15860c1d0a8107a3a151db8
         with:
           github-token: ${{ secrets.FLUTTERGITHUBBOT_TOKEN }}
-          source: 'master'
-          dest: 'main'
+          source: 'main'
+          dest: 'master'