home-environment: add home.uid option

Add a home.uid option similar to home.username. When set, the
activation script verifies the current UID matches the expected
value using the new checkUid function.

When using the NixOS or nix-darwin modules, home.uid is
auto-discovered from users.users.<name>.uid when that value
is set.

This is useful for constructing paths that depend on the user's
UID, such as /run/user/<uid> paths for gpg-agent sockets or
other user-specific runtime directories.

nixos/common.nix

@@ -53,6 +53,7 @@ let
 
             home.username = config.users.users.${name}.name;
             home.homeDirectory = config.users.users.${name}.home;
+            home.uid = mkIf (config.users.users.${name}.uid != null) config.users.users.${name}.uid;
 
             # Forward `nix.enable` from the OS configuration. The
             # conditional is to check whether nix-darwin is new enough