sources: update nix/hexchen

this contains random changes, but most notably the fix for #9.
(upstream
45d6c3bfed)

nix/sources.json

@@ -57,8 +57,9 @@
     },
     "nix-hexchen": {
         "branch": "main",
+        "ref": "main",
         "repo": "https://gitlab.com/hexchen/nixfiles.git",
-        "rev": "2dfbd032f05ad79176a5756cd76dbda4a828e4d8",
+        "rev": "1dee1474f147ce8d04ec4b200df1c6255ae59bdc",
         "type": "git"
     },
     "nixos-mailserver": {