feat(ci): build with nixos

.gitlab-ci.yml

-image: "debian"
-
 build:
     stage: build
+    image: debian
     before_script:
         - echo "deb http://deb.debian.org/debian stretch-backports main" >> /etc/apt/sources.list
         - apt update -qq
@@ -11,3 +10,9 @@ build:
     script:
         - ./bootstrap.sh
         - ninja -C build/
+
+nix:
+    stage: build
+    image: nixos/nix:latest
+    script:
+        - nix-build