ci: run checks on all branches

.github/workflows/rust.yml

@@ -1,10 +1,6 @@
 name: Rust
 
-on:
-  push:
-    branches: [main]
-  pull_request:
-    branches: [main]
+on: [push, pull_request]
 
 concurrency:
   # Documentation suggests ${{ github.head_ref }}, but that's only available on pull_request/pull_request_target triggers, so using ${{ github.ref }}.