ci(spack): add concurrency group to auto-cancel stale runs

Picks up the latest spack-packages add-mfc tip (now includes the
python-venv build dep) and saves CI minutes by cancelling in-progress
runs on the same branch when a new push lands.

Author: sbryngelson

.github/workflows/spack.yml

@@ -5,6 +5,10 @@ on:
     schedule:
         - cron: '17 3 * * 0'
 
+concurrency:
+    group: ${{ github.workflow }}-${{ github.ref }}
+    cancel-in-progress: true
+
 jobs:
     spack-install:
         name: Spack install + smoke test