xtask: migrating from duct to xshell

[gabriele-0201]

Duct exhibits too many peculiar behaviors:

• Regarding how it deals with relative paths, it seems that the spawned process assumes a position in the file system relative to where the parent process was spawned, not where the child process was spawned.
• You cannot use the same opened file in multiple commands because they take ownership of the file descriptor and close it once the command finishes.
• If you capture stderr to print it later but the command fails with a non-zero status code, it will drop the stderr and return an empty enum. Stdout and stderr are only returned if the command terminates correctly.
• There is no way to convert duct::Cmd to a std::process::Command that is needed to deal with process group IDs.

That's why I am moving to xshell. However, it does not handle async, so the addition of Tokio is required. Moving everything to Tokio raises all the problems related to issue #228. I handled them using the approach that @pepyakin and I came up with. Zombie processes should not be a problem anymore

closes #228

[gabriele-0201]

Current dependencies on/for this PR:

• xtask: migrating from duct to xshell #251 👈
• ci: print logs if xtask tests fail #264
• main

This stack of pull requests is managed by Graphite.

[rphmeier]

this approach looks much more robust