With an addition of useFetchCargoVendor, crane becomes less necessary
for our use. This reuses the package from nixpkgs as well as creating a
better devshell that both work on macOS.
I use Xcode's SDKROOT and DEVELOPER_DIR to point the swift in the
livekit client crate to a correct sdk when using a devshell. Devshell
should work without that once apple releases sources for the 15.1 SDK
but for now this is an easy fix
This also replaces fenix with rust-overlay because of issues with the
out-of-sandbox access I've noticed fenix installed toolchains have
Release Notes:
- N/A
