refactor(ops/nixos): Move my NixOS configurations to //users/tazjin

NixOS modules move one level up because it's unlikely that //ops/nixos
will contain actual systems at this point (they're user-specific).

This is the first users folder, so it is also added to the root
readTree invocation for the repository.

Change-Id: I546c701145fa204b7ba7518a8a56a783588629e0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/244
Reviewed-by: tazjin <mail@tazj.in>

default.nix

@@ -41,6 +41,7 @@ let
     presentations = readTree ./presentations;
     third_party   = readTree ./third_party;
     tools         = readTree ./tools;
+    users         = readTree ./users;
     web           = readTree ./web;
   };
 in fix(self: {