Add CI concurrency group (#1435)

### What

Add CI concurrency group limiting to one build per branch, excluding
main, canceling any older runs.

### Why

Limit CI resource usage. We never need out-of-date commit CI runs to
complete on PRs. We do need to make sure that release vs pr builds vs
merge group build runs run to completion independently so that is
included in the identifier for the concurrency group.

.github/workflows/rust.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
   merge_group:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_protected == 'true' && github.sha || github.ref }}-{{ github.event_name }}
+  cancel-in-progress: true
+
 jobs:
 
   complete: