add nix: apkg + CI

This depends on MR !1188 (merged) (includes it). Otherwise I think it's merge-able, but there's no need to hurry as it's mainly meant a proof of concept.

ci/pkgtest.yaml

@@ -131,6 +131,25 @@ ubuntu-21.04:pkgbuild:
     DISTROTEST_REPO: xUbuntu_21.04
   script:
     - *debpkgbuild
+
+nixos-unstable:pkgbuild:
+  <<: *pkgbuild
+  # We do NOT use LXC, for now at least.
+  tags:
+    - docker
+    - linux
+    - amd64
+  image: nixos/nix
+
+  variables:
+    #NIX_PATH: nixpkgs=https://github.com/nixos/nixpkgs/archive/nixos-unstable.tar.gz
+    NIX_PATH: nixpkgs=https://github.com/vcunat/nixpkgs/archive/p/apkg.tar.gz
+  before_script:
+  script:
+    - nix build nixpkgs.apkg
+    # the image auto-detects as alpine distro
+    - ./result/bin/apkg install -d nix
+    - kresd --version
 # }}}
 
 # pkgtest {{{