Implement caching for /nix/store

.github/workflows/ci.yml

@@ -32,6 +32,14 @@ jobs:
             - name: Sync repository
               uses: actions/checkout@v4
 
+            - name: Set up `/nix/store` cache
+              uses: actions/cache@v3
+              with:
+                  path: /nix/store
+                  key: ${{ runner.os }}-nix-store-${{ hashFiles('flake.lock') }}
+                  restore-keys: |
+                      ${{ runner.os }}-nix-store-
+
             - name: Install Nix (with flakes and nix-command enabled)
               uses: cachix/install-nix-action@v26
               with:
@@ -65,7 +73,7 @@ jobs:
                   nix develop --command true
                   direnv allow
 
-            - name: Perform continuous integration
+            - name: Run CI tests
               run: |
                   direnv exec . engage