add guix-based pipeline building a tarball

the build artefacts should be relocatable and run on (almost?) every
system that has a linux kernel underneath.

.gitlab-ci.yml

+image: registry.stuebinm.eu/guix
+
+stages:
+  - build
+
+build-job:
+  stage: build
+  script:
+    - cp $(guix pack -m manifest.scm -f tarball -S /bin=bin --relocatable --verbosity=3) walint.tar
+  artifacts:
+    paths:
+      - walint.tar