nixos/installer: Override swap devices so tests don't fail

This patch should be reverted if either:
  - systemd fixes the multi-swapon issue.
    https://bugs.freedesktop.org/show_bug.cgi?id=86930
  - If we disable the autogeneration of swap and vfat units within
    systemd.

nixos/tests/installer.nix

@@ -79,6 +79,8 @@ let
       virtualisation.pathsInNixDB = channelContents ++ [ pkgs.hello.src ];
       virtualisation.memorySize = 768;
 
+      swapDevices = mkOverride 0 [ ];
+
       networking.firewall.allowedTCPPorts = [ 80 ];
     };