feat: add lint check to CI

.gitlab-ci.yml

 # Fixing the nixos image saves CI time so it doesn't have to pull new
-#image every time (nixos/nix updates quite often).
+# image every time (nixos/nix updates quite often).
 # image: nixos/nix:latest
 image: nixos/nix:2.24.8
 
-#before_script:
-  #- nix-env -iA nixpkgs.nix nixpkgs.cacert
-  #- apt-get update
-  #- apt-get install make xz-utils
-
 stages:
   # - deps
   - compile
   - test
+  - lint
 
 # deps:
 #   stage: deps
@@ -38,8 +34,6 @@ compile:
 test:
   stage: test
   cache:
-    # cache per branch name
-    # key: ${CI_COMMIT_REF_SLUG}
     paths:
       - /nix/store
       - node_modules/
@@ -51,3 +45,15 @@ test:
     - nix --extra-experimental-features "nix-command flakes" run .#test-ps
     
     - nix-collect-garbage --delete-older-than 14d
+
+lint:
+  stage: lint
+  cache:
+    paths:
+      - /nix/store
+      - node_modules/
+      - output/
+      - .spago/
+  
+  script:
+    - nix --extra-experimental-features "nix-command flakes" run .#purs-tidy -- check src