workflow: change nightly default build branch

Author: amazingfate

.github/workflows/nightly.yml

@@ -23,7 +23,7 @@ jobs:
         uses: adriangl/check-new-commits-action@v1
         with:
           seconds: 86400 # One day in seconds
-          branch: 'master'
+          branch: 'master-ex'
 
       - name: Cancel action if no new commits
         if: ${{ steps.check-new-commits.outputs.has-new-commits != 'true' }}