ci(spack): use spack/setup-spack with buildcache

Replaces the manual spack + spack-packages clone + cache plumbing with
the official spack/setup-spack@v3 action. buildcache: true pulls
pre-built binaries from spack/github-actions-buildcache for common deps
(cantera, boost, fftw, openblas, ...), cutting cold-cache wall time
from ~1.5h to minutes when cache hits are available.

Author: sbryngelson

.github/workflows/spack.yml

@@ -12,37 +12,20 @@ jobs:
         steps:
           - uses: actions/checkout@v4
 
-          - name: Check out Spack
-            uses: actions/checkout@v4
+          - name: Set up Spack with buildcache
+            uses: spack/setup-spack@v3
             with:
-                repository: spack/spack
-                path: spack
-
-          - name: Check out spack-packages (sbryngelson/add-mfc until upstream merges)
-            uses: actions/checkout@v4
-            with:
-                repository: sbryngelson/spack-packages
-                ref: add-mfc
-                path: spack-packages
-
-          - name: Cache Spack store
-            uses: actions/cache@v4
-            with:
-                path: |
-                    spack/opt
-                    spack/var/spack/cache
-                key: spack-mfc-${{ runner.os }}-${{ hashFiles('spack-packages/repos/spack_repo/builtin/packages/mfc/package.py', 'spack-packages/repos/spack_repo/builtin/packages/cantera/package.py', 'spack-packages/repos/spack_repo/builtin/packages/py_pyrometheus/package.py') }}
-                restore-keys: spack-mfc-${{ runner.os }}-
+                # Pin to the sbryngelson/add-mfc branch until upstream
+                # spack/spack-packages merges the mfc recipe.
+                packages_repository: sbryngelson/spack-packages
+                packages_ref: add-mfc
+                buildcache: true
 
           - name: Install mfc+chemistry~mpi
-            run: |
-                source spack/share/spack/setup-env.sh
-                spack repo add --scope=site spack-packages/repos/spack_repo/builtin
-                spack install --reuse mfc+chemistry~mpi
+            run: spack install --reuse mfc+chemistry~mpi
 
           - name: Smoke test
             run: |
-                source spack/share/spack/setup-env.sh
                 spack load mfc
                 prefix="$(spack location -i mfc)"
                 echo "MFC prefix: $prefix"
@@ -52,8 +35,6 @@ jobs:
                 done
                 test -x "$prefix/bin/mfc" || { echo "missing shim: mfc"; exit 1; }
                 echo "found shim: $prefix/bin/mfc"
-                # Validate a small example case file through MFC's toolchain
-                # (exercises Python venv-skip + PYTHONPATH wiring under MFC_PREBUILT_PREFIX).
                 cd "$prefix/share/mfc"
                 ./mfc.sh run --prebuilt-prefix="$prefix" \
                              --no-mpi --dry-run \