chore(ci): add check for building the nix flake

.gitlab-ci.yml

@@ -9,3 +9,10 @@ unittests:
     - redo check
   tags:
     - docker-alpine
+
+nix-build:
+  stage: test
+  image: nixpkgs/nix:nixos-21.11
+  script: nix build '.#atools'
+  tags:
+    - ci-build