* single-instance: fix `cwd` in single instance on macOS
which was the `cwd` of the first instance, instead of the second how it should be and is on windows and linux.
also add rustfmt.toml to enforce the correct formatting (4 spaces for indent)
* use split_once
* remove rustfmt
* fix indentation
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Co-authored-by: Lucas Nogueira <lucas@tauri.app>
Simon Laux