CI: Try to use the GitHub cache for the Guix store

Unfortunately, Guix does not have anything comparable to Cachix,
therefore caching the build results might need some other tricks.

.github/workflows/ci.yml

@@ -99,21 +99,53 @@ jobs:
 
     runs-on: ${{ matrix.os }}
     steps:
-      - name: Guix cache
-        uses: actions/cache@v3.3.2
+      - name: Prepare the Guix store before restoring from cache
+        working-directory: .
+        run: |
+          # Make the Guix store writable for the current user
+          sudo mkdir -p /gnu/store /var/guix/db /var/guix/gcroots /var/guix/profiles
+          sudo chown -R ${USER}: /gnu /var/guix
+          sudo chmod -R u+w /gnu /var/guix
+
+      - name: Restore the Guix store from cache
+        id: restore_cache
+        uses: actions/cache/restore@v3.3.2
         with:
-          path: ~/.cache/guix
-          key: guix-cache-${{ github.sha }}
+          path: |
+            ~/.cache/guix
+            /gnu
+            /var/guix/db/db.sqlite
+            /var/guix/gcroots
+            /var/guix/profiles
+          key: guix-store-${{ github.sha }}-${{ github.run_id }}
           restore-keys: |
-            guix-cache-
+            guix-store-${{ github.sha }}-
+            guix-store-
+
+      - name: Fix up permissions for the Guix store
+        working-directory: .
+        run: |
+          # Fix up permissions for the Guix store when restoring from cache
+          sudo chown -R root:root /gnu /var/guix
+          sudo chmod 755 /gnu
+          sudo chmod -R u-w /gnu/store
+          sudo chmod 1775 /gnu/store
 
       - name: Install Guix
-        uses: sigprof/guix-install-action@0fe71536357cf2a15be1960cd2dbce2cf89ba06a
+        uses: sigprof/guix-install-action@65e5b79e7f00e3f72ea1beb13431e036cc8ca2d2
+        with:
+          useExistingGuix: ${{ steps.restore_cache.outputs.cache-matched-key != '' }}
+          pullAfterInstall: false
 
       - name: Ensure no locale warning
+        id: guix_setup_check
         working-directory: .
         run: test -z "$(guix --version 2>&1 >/dev/null)"
 
+      - name: Fail
+        working-directory: .
+        run: false
+
       - name: Checkout the project source
         uses: actions/checkout@v4.1.1
         with:
@@ -173,6 +205,26 @@ jobs:
       - name: Test 'qmk clean'
         run: guix shell -m ../guix-qmk/manifest.scm -- qmk clean
 
+      - name: Prepare to save the Guix store
+        id: guix_shutdown
+        if: always() && !cancelled() && (steps.guix_setup_check.outcome == 'success')
+        working-directory: .
+        run: |
+          sudo systemctl stop guix-daemon.service
+          sudo systemctl stop gnu-store.mount
+
+      - name: Save the Guix store in the cache
+        if: always() && !cancelled() && (steps.guix_shutdown.outcome == 'success')
+        uses: actions/cache/save@v3.3.2
+        with:
+          path: |
+            ~/.cache/guix
+            /gnu
+            /var/guix/db/db.sqlite
+            /var/guix/gcroots
+            /var/guix/profiles
+          key: guix-store-${{ github.sha }}-${{ github.run_id }}
+
   finish:
     needs:
       - setup