Support building on macOS using Nix Shell

[ljrk0]

I could build verso ontop of servo using my rustup.sh install on macOS AARCH64, but supplying the other dependencies neatly using Nix Shell:

$ nix-shell -p pkg-config harfbuzz libiconv cmake \
  darwin.apple_sdk.frameworks.{CoreServices,AppKit,IOBluetooth} --run 'cargo run'

Probably a similar invocation can be used to build servo using /etc/shell.nix but that's very hard-coded to NixOS AFAICT and I'm too much of a Nix noob to adjust the Nix script to just pull in the above deps + Rust.

[mrobinson]

There's really only two dependencies that you need to build Servo on macOS, cmake and pkg-config. I'm not sure if it makes sense to maintain an entire nix build for macOS for two dependencies.

[delan]

We may be able to do this with some small tweaks to the existing shell.nix. @ljrk0, can you tell me more about the problems you ran into when using our shell.nix on macOS?