Add caching and fix build issues in Spack CI

Changes:
1. Add GitHub Actions cache for Spack dependencies
   - Caches spack/opt/spack (installed packages)
   - Caches spack/var/spack/cache (downloaded sources)
   - Key based on package.py hash for cache invalidation
   - Dramatically speeds up subsequent CI runs

2. Split dependency installation into separate step
   - Install dependencies first (these benefit from cache)
   - Install MFC separately for clearer CI output
   - Reduced timeouts since cached builds are much faster

3. Use mfc@master instead of v5.1.0
   - v5.1.0 has parallel build dependency issues with m_thermochem
   - Master branch has fixes for parallel builds
   - More representative of what users will install

Expected improvement: ~70min -> ~10min on cache hit

Author: sbryngelson

.github/workflows/spack.yml

@@ -145,6 +145,16 @@ jobs:
           spack external find --not-buildable cmake python perl gfortran gcc
           spack config add "packages:all:target:[x86_64]"
 
+      - name: Cache Spack Dependencies
+        uses: actions/cache@v4
+        with:
+          path: |
+            spack/opt/spack
+            spack/var/spack/cache
+          key: spack-deps-${{ runner.os }}-${{ hashFiles('packaging/spack/package.py') }}
+          restore-keys: |
+            spack-deps-${{ runner.os }}-
+
       - name: Install Package into Spack
         run: |
           . spack/share/spack/setup-env.sh
@@ -157,13 +167,20 @@ jobs:
           # Verify package is visible
           spack list mfc
 
+      - name: Install MFC Dependencies
+        run: |
+          . spack/share/spack/setup-env.sh
+          # Install just the dependencies first (these are cached)
+          spack install --show-log-on-error --only dependencies mfc@master~mpi~post_process
+        timeout-minutes: 35
+
       - name: Install MFC via Spack
         run: |
           . spack/share/spack/setup-env.sh
-          # Install minimal configuration
-          # Binary cache is already configured, Spack will use it automatically when available
-          spack install --show-log-on-error mfc~mpi~post_process
-        timeout-minutes: 45
+          # Install MFC from master branch (v5.1.0 has parallel build issues)
+          # This step is fast since dependencies are already installed
+          spack install --show-log-on-error mfc@master~mpi~post_process
+        timeout-minutes: 10
 
       - name: Test MFC Execution
         run: |