Add CI cache

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

.gitlab-ci.yml

@@ -1,8 +1,19 @@
 stages:
   - build
 
+cache: &global_cache
+  key:
+    files:
+      - flake.lock
+      - flake.nix
+  paths:
+    - /nix/store
+  policy: pull-push
+
 build:
   image: nixos/nix
   stage: build
+  cache:
+    <<: *global_cache
   script:
     - nix --experimental-features 'nix-command flakes' flake check