I made a small change to jellyfin-mpv-shim in nixpkgs such that it installs on darwin. However, when I try to test it by building via nix-build -A jellyfin-mpv-shim, nix tries to build many packages and takes a very long time. Is this expected? All I did was remove a couple dependencies.
The commit: Restrict pystray and tkinter to linux · NixOS/nixpkgs@a9bfd0e · GitHub
Your change is against the latest commit of the master branch, for which packages might not have been built and uploaded to the nix cache yet, depending on what are the latest changes there.
You can try applying your commit against the nixpkgs-unstable or nixos-unstable branches instead, most packages of those branches will be in the nix cache. Those branches are always lagging behind master (N commits behind).