Some environment variables, such as `RUST_LOG` include equals signs. Nushell
should support this in the shorthand environment variable syntax so that
developers using these variables can control them easily. We accomplish this by
swapping `std::str::split` for `std::str::splitn`, which ensures that we only
consider the first equals sign in the string instead of all of them, which we
did previously.
Closes#3867
762e528ec5