sources: update nix-hexchen

lots of fancy new stuff, but most importantly: we no longer import all
of my user config, just the very base.

none of that fancy stuff is active right now, this should mostly be a
no-op unless we do the same restructure that i have just done in my
nixfiles here as well.

common/hexchen.nix

@@ -4,6 +4,6 @@ let
   sources = import ../nix/sources.nix;
 in {
   imports = [
-    (sources.nix-hexchen + "/users/hexchen")
+    (import sources.nix-hexchen).users.hexchen.base
   ];
 }

nix/sources.json

@@ -27,7 +27,7 @@
         "branch": "main",
         "ref": "main",
         "repo": "https://gitlab.com/hexchen/nixfiles.git",
-        "rev": "029493c1ad1e30007b25ac37bb09fdc75b4108e3",
+        "rev": "2ed640ef19f80bda235c75f51c8be80e8a675304",
         "type": "git"
     },
     "nixos-mailserver": {