Draft: Fix the Gitlab CI pipeline

Wrap all cabal calls into the development nix environment.

.gitlab-ci.yml

-image: nixos/nix
+image: lnl7/nix
 
 workflow:
   rules:
@@ -8,11 +8,9 @@ ci:
   tags:
     - x86_64-linux
   script:
-    - nix-shell -p git
-    - nix-shell
-    - cabal update
-    - cabal build debugger
-    - cabal test
+    - nix-shell --pure --command "cabal update"
+    - nix-shell --pure --command "cabal -v build all"
+    - nix-shell --pure --command "cabal test all"
 
 pages:
   script: