Stabilise Command::raw_arg

[passcod]

raw_arg in std is stable from Rust 1.62, and Tokio 1.30's MSRV is 1.63.

The note in Tokio's raw_arg (added in #5704) documentation says:

Note: This is an unstable API but will be stabilised once tokio’s MSRV is sufficiently new.

I think that's the case now :)

[Darksonn]

I'm that case, a PR world be appreciated. Otherwise I can look at it later.