`current_dir` and relative paths to executables works differently on
unix and windows (unix behavior does not make sense), see:
17e30e83a1/src/lib.rs (L295-L324)
The original motivation to set cwd was to make rustfmt read the
correct rustfmt.toml, but that was future proofing, rather than a bug
fix.
So, let's just remove this and see if breaks or fixes more use-cases.
If support for per-file config is needed, we could use `--config-path`
flag.
ffe4a5e114