Add cache for NixOS builds

.github/workflows/ci.yml

@@ -8,6 +8,9 @@ jobs:
 
   test:
     runs-on: ubuntu-latest
+    container:
+      volumes:
+        - /mnt/cache/nix_store:/nix
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
@@ -22,6 +25,9 @@ jobs:
 
   deploy:
     runs-on: ubuntu-latest
+    container:
+      volumes:
+        - /mnt/cache/nix_store:/nix
     needs: test
     if: github.ref == 'refs/heads/master'
     steps:

.github/workflows/update.yml

@@ -6,6 +6,9 @@ on:
 jobs:
   update-lockfile:
     runs-on: ubuntu-latest
+    container:
+      volumes:
+        - /mnt/cache/nix_store:/nix
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4