[Feature/Enhancement]Provide cachix server for NixOS flake users

[Raviexthegod]

One big thing that gets me to somewhat dislike using Millennium on my primary machine, which I run NixOS on, is that since there isn't a cache available where Hydra has already built the package, at least as far as I'm aware, it drastically increases rebuild times every time Millennium updates as it forces me to completely rebuild the entire package, including the Steam-Unwrapped shell, the millennium injector, and the FHS environment they reside in, usually causing rebuilds to take an extra 10 minutes at a time, where my CPU has all cores pinned at 100% utilization and full clock speed. For this reason, I feel like it would be beneficial to set up a hydra server that will automatically build every new version of the package and provide that build to end-users computers when their local nix instance requests it upon rebuilding and noticing the new hash as a result of the update. It would also provide an automated way to test each version with the Nix packaging, that way if something does result in a failure to build for whatever reason, Hydra will throw an error that I believe will notify package maintainers that something's broken.