Reduce GitHub Actions usage

This removes the run on push for branches other than main, and
preemptively stops runs on the same ref

.github/workflows/ci.yaml

@@ -2,9 +2,14 @@ name: CI
 
 on:
   push:
+    branches: [ main ]
   pull_request:
     branches: [ main ]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   CARGO_TERM_COLOR: always