Use env_clear for environment variable reset

[cd-work]

We're already using the Command interface on all platforms since we need to spawn a separate process on Linux. However we're still manually removing the environment variables from the process which seems fragile.

Instead, we should use the Command::env_clear method instead:
https://doc.rust-lang.org/std/process/struct.Command.html#method.env_clear

This removes all inherited environment variables, after which they can be added again with Command::env and Command::envs. The inherited variables stay cleared.

This also works on Linux with the new PID 1 init, even if PID 1 itself still has these variables set, since PID 2 cannot read /proc/1/environ based on my testing.