miri-script and cargo-miri cleanups

[RalfJung]

At least in an initial test, cmd!(Shell::new()?, "env") does show the full environment. Let's see what CI says.

@osiewicz why did you add this code to forward the host environment?

[osiewicz]

Hi,
Yeah, it might be redundant given that it was added quite early in the migration to xshell and not everything around environment variables worked fine back then. This was something I wanted to tackle during the final touchups to the Miri-script PR, though I was more-so focused on making CI green first.

[RalfJung]

Okay then, let's get rid of it. :) Please let me know if there is any other cleanup you had pending. :D

Also, this finally gets rid of that pesky set_var in cargo-miri. It wasn't even that hard. ;) Cc #2783

@bors r+

[bors]

📌 Commit f96c30d has been approved by RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit f96c30d with merge 606adf9...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 606adf9 to master...