CI: Build spack build on pull requests

Author: gsjaardema

.github/workflows/spack.yml

@@ -6,6 +6,9 @@ on:
   push:
     branches:
     - master
+  pull_request:
+    branches:
+    - master
 
 concurrency:  
   group: ${{ github.workflow}}-${{ github.head_ref }}