flake: infer default.nix import path (#1636)

Re-apply commit 0f93e58628 ("flake: infer default.nix import path
(#1544)") that was accidentally reverted in commit a5c1532ab8 ("flake:
partition dev inputs (#1289)").

Link: https://github.com/nix-community/stylix/pull/1636

Reviewed-by: awwpotato <awwpotato@voidq.com>

flake/dev/packages.nix

@@ -10,7 +10,7 @@
       # Testbeds are virtual machines based on NixOS, therefore they are
       # only available for Linux systems.
       packages = lib.mkIf pkgs.stdenv.hostPlatform.isLinux (
-        import ../../stylix/testbed/default.nix {
+        import ../../stylix/testbed {
           inherit pkgs inputs lib;
         }
       );