Panics when any environment variable contains non-unicode characters

[printfn]

This crate causes a panic if any environment variable contains non-unicode characters, such as when PWD points at a non-unicode directory, e.g. when running mkdir $'\xff'.

https://github.com/mehcode/config-rs/blob/a38387955622e6b8d48a49352629cd329ad6dc29/src/env.rs#L239

This line needs to be changed to env::vars_os(), with appropriate Unicode escaping and/or filtering of unencodable environment variables.

Downstream issue: atuinsh/atuin#2428