Added method as_std_mut to tokio::process::Command

[tglane]

Motivation

Citing from #6523:

Sometimes I want to access a mutable reference of std::process::Command out of the tokio counterpart, so I can access some unstable functions in std to update Command, but tokio only provides an as_std method returning a shared reference.

Solution

I added a method as_std_mut(&mut self) -> &mut std::process::Command to tokio::process::Command to get mutable access to the inner std command structure.

This closes #6523

[Darksonn]

Thanks.