gitlab pipeline: guix → debian-based image + stack

.gitlab-ci.yml

-image: registry.stuebinm.eu/guix
+image: haskell
 
 stages:
   - build
@@ -6,7 +6,7 @@ stages:
 build-job:
   stage: build
   script:
-    - cp $(guix pack -m manifest.scm -f tarball -S /bin=bin --relocatable --verbosity=3) walint.tar
+    - stack build --install-ghc
   artifacts:
     paths:
-      - walint.tar
+      - .stack-work/install/x86_64-linux-tinfo6/*/*/bin/walint