CI: Don't run on push except on main and stable branches

Right now, when opening a PR the CI is ran twice (one for the push event and one for the PR event).

This prevents that: PRs will only get the CI ran once

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

Author: AlvaroBrey

.github/workflows/android.yml

@@ -1,6 +1,9 @@
 name: Android CI
 
-on: [push, pull_request]
+on:
+  pull_request:
+  push:
+    branches: [ main, stable-* ]
 
 permissions:
   contents: read