simplify CI

* I didn't know what I was doing when I set up Hydra

* it's not certain that I do now either, but hey ho

tests/pppoe/test.nix

@@ -1,9 +1,5 @@
-{
-  liminix
-, nixpkgs
-}:
-let img = (import liminix {
-      device = import "${liminix}/devices/qemu";
+let img = (import <liminix> {
+      device = import <liminix/devices/qemu>;
       liminix-config = ./configuration.nix;
     }).outputs.default;
     pkgs = import <nixpkgs> { overlays = [(import ../../overlay.nix)]; };