Try to use uname from nix or libc crates

[stanislav-tkach]

Perhaps it would be better to use a function from one of these crates instead of invoking the uname process.

[sunshowers]

Sadly I learned that uname -o doesn't have a (portable?) programmatic API. Other uses of uname can probably be replaced though.

[sunshowers]

I did find https://docs.rs/platform-info/latest/platform_info/, which hardcodes the OS: https://docs.rs/platform-info/latest/src/platform_info/lib_impl.rs.html