Update .gitlab-ci.yml

.gitlab-ci.yml

-image: nixpkgs/nix:latest
+image: registry.stuebinm.eu/guix
 
 stages:
   - build
@@ -6,8 +6,7 @@ stages:
 build-job:
   stage: build
   script:
-    - nix-build build.nix
-    - cp result/bin/walint .
+    - cp $(guix pack -m manifest.scm -f tarball -S /bin=bin) walint.tar
   artifacts:
     paths:
-      - walint
+      - walint.tar