Update Nix workflow to open a PR

[zachallaun]

Context: #709 (comment)

[scohen]

@zachallaun i gave the github token more access, let's see if it works.

[skbolton]

@scohen just a heads up the last run of the bot failed due to branch protection rules kicking in

[skbolton]

Also someone else has started packaging lexical in the official nixpkgs repository and are just bumping it along as new releases occur. I think this works better than trying to build alongside main since it seems that non of the core maintainers use nix or check its build. I think it would be safe to remove all the nix definitions from this repo.