nixos/kresd: Fix unsupported regex

[griff]

Regex supported by builtins.match differ on Linux and Darwin
(see NixOS/nix#1537) and the empty match group errors on Darwin.
But simply removing it does not change the logic in the module in any
way.

Motivation for this change

The kresd module can't at the moment be evaluated on Darwin due to NixOS/nix#1537 since empty match groups aren't supported there.

Things done

Simply removed empty match group since all the rest of the logic works exactly the same with or without it.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[doronbehar]

Diff LGTM, please resolve merge conflicts.

[vcunat]

BTW, you don't actually use NixOS modules on darwin, right? (just eval for *-linux)

[vcunat]

Looks good to me. I don't have any darwin at hand; I'll trust you on that.

[vcunat]

Thanks for mentioning me.

[griff]

@vcunat yes I just eval for *-linux and then have a VM that is a remote builder.