ci: cancel superseded rust-ci runs on pull requests (#4555)

.github/workflows/rust-ci.yaml

@@ -7,6 +7,10 @@ on:
     branches: [ main ]
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 defaults:
   run:
     working-directory: rustutils/