chore: run github actions on pull requests and master

- If we run them on all pull requests and pushes, we'll run them twice.
- If we run them only on push, we won't run them for forks.

Basically: If you want CI, open a PR.

Author: Stebalien

.github/workflows/ci.yml

@@ -2,6 +2,9 @@ name: Continuous integration
 
 on:
   push:
+    branches:
+      - master
+  pull_request:
 
 env:
   RUSTFLAGS: -Dwarnings