Fix streaming stdout/stderr when passing stdin to child process

[uovobw]

from shell.rs:

This method is needed because we cannot stream stdout / stderr when stdin
handling is involved. We have 0 clues why, we are not smart enough to understand
tokio in its fullness.

Ideas: tokio-rs/tokio#2199

So we implemented this non streaming method to overcome the limitation and thus be
able to generate Wireguard private and public key.
This might be automatically solved when we move to tokio 1 or when someone smarter
comes around and fix this.