ci: turn on workflow for all PR

Due to the squash policy, PRs will be more granular, so there will be a situation of chains of PRs
going into each other. To make it easier to work with this, you can enable CI on any PR. For an
open-source project this will not be expensive.

.github/workflows/rust.yml

@@ -4,9 +4,7 @@ on:
   push:
     branches:
       - main
-  pull_request:
-    branches:
-      - main
+  pull_request
 
 env:
   CARGO_TERM_COLOR: always