Change CI to shell runner

.gitlab-ci.yml

-image: nixos/nix:latest
-
-before_script:
-- export LC_ALL=C.UTF-8
-- nix-channel --add https://nixos.org/channels/nixpkgs-unstable nixpkgs
-- nix-channel --update
-- nix-env -iA nixpkgs.stack
-- nix-env -iA nixpkgs.hlint
-
 stages:
 - lint
 - build
 
-hlint:
+lint:
   stage: lint
   script:
   - hlint .
@@ -19,4 +10,4 @@ hlint:
 build:
   stage: build
   script:
-  - stack --nix build --ghc-options "-Wall -Werror"
+  - stack --nix test