Millennium Steam breaking

Looking for a bit of help here. I'm trying to install Millennium for Steam using this guide however when I attempt to change the progrms.steam.package field to pkgs.millennium, which is added by the overlay I set in my configuration.nix, it errors out saying it's calling an invalid argument extraENV, even though when I look at the file it's blaming I see no references to said argument, including the file in the Nix store.

Screenshot of the error

Apologies for including a screenshot instead of the actual text, I use the nh command which has some formatting that doesn't work without nerdfonts, which Reddit obviously doesn't suppoort.

I tried rebuilding with just the millennium input and the nixpkgs overlay to see if adding it to the store first and switching to that configuration would fix it, but it still gives the same error upon rebuilding with the changed flag. If you want to reference my configuration, it can be found here, and the commit containing the offending code can be found here. I committed it figuring it would help with the troubleshooting process, just had to add a note to self to not use that version to rebuild.