CI: always run the linux workflow

This makes it easier for contributors to check if their code runs correctly
without having to file a PR.

The Mac and Windows workflows are a bit more involved, so let's keep those on
pull requests only.

.github/workflows/linux.yml

@@ -1,10 +1,6 @@
 name: linux
 
-on:
-  push:
-    branches: [ master ]
-  pull_request:
-    branches: [ master ]
+on: [push, pull_request]
 
 # Set permissions at the job level.
 permissions: {}