fix(pm): don't fail evaluation on multiple homes

As a regression from some of the assertion refactoring...

When you have multiple homes, we were previously comparing the entire
config of each of them to all of the rest. This was a Bad Thing as it
caused us to evaluate the whole of each home, including some attributes
that normally aren't evaluated and broke evaluation completely.

This was relevant for Maya but not for the rest of us - I'm not sure how
CI missed it...

packetmix/modules/nilla-home/nixos.nix

···
                 (
                   values:
                   let
-                    existingUsernames = builtins.filter (value: value.username != null) values;
+                    existingUsernames = map (value: value.username) (
+                      builtins.filter (value: value.username != null) values
+                    );
                     uniqueUsernames = lib.lists.unique existingUsernames;
                   in
                   if existingUsernames != uniqueUsernames then