switch pppoe test to use armv7l

tests/pppoe/test.nix

@@ -3,7 +3,7 @@
 , nixpkgs
 }:
 let img = (import liminix {
-      device = import "${liminix}/devices/qemu/";
+      device = import "${liminix}/devices/qemu-armv7l";
       liminix-config = ./configuration.nix;
     }).outputs.default;
     pkgs = import <nixpkgs> { overlays = [(import ../../overlay.nix)]; };