Add Command::env_clear method

[jsha]

std::process:Command has an env_clear method to clear all environment variables before forking a process. As far as I can tell, it's not possible to call this on rustwide::cmd::Command. It would be nice to add it. In particular, docs.rs uses rustwide::cmd::Command to run cargo fetch in a non-containerized process; clearing the environment would be one simple way to reduce the possibility of accidentally propagating host configuration. See rust-lang/docs.rs#1881 for an example.