Github Actions: Use the new default branch macro.

Trigger deployment on pushes to the default branch instead of
hard-coding `master`. This will keep ci running as expected if
the default branch is changed, e.g. to the friendlier `main`
convention.

.github/workflows/deploy.yml

@@ -2,7 +2,7 @@ name: 'Push'
 on:
   push:
     branches:
-      - master
+      - $default-branch
 jobs:
   build:
     runs-on: ubuntu-latest