Meta: Add `Guix` tag

It is viewed as just a nix fork with scheme on top of in many communities I am in, while yes it might be started off as that but it has grown significantly in the recent years that it should be treated as its own project. I have been just in guix community for like year and so but I have seen very good changes, some of them would The new rust build system Moving to Codeberg (this brought many new contributors) new v1.5.0 release :) But what I like most about the guix project is the people there, It is small but very tightly-knit community. So I believe guix deserve its own tag now.