Temporarily build the nicholson/buildsystem branch in CI, rather than develop.

Author: nkoukpaizan

.github/workflows/spack_default_build.yaml

@@ -96,7 +96,7 @@ jobs:
       matrix:
         # Minimal Build(s) - GHCR mirror speeds these up a lot!
         spack_spec:
-          - gridkit@develop
+          - gridkit@buildsystem
 
     name: Build gridkit with Spack
     steps: