Add builders to verify that packaging rust in a typical configuration works

[Mark-Simulacrum]

This is intended to prevent issues such as #42543. Ideally, we would run this as a Travis bot -- probably doesn't need to run tests, but uncertain.

Specifically, --enable-local-rust.

[est31]

In such an environment, we should also make sure that we don't have network access, to prevent bugs like #42719.

[Mark-Simulacrum]

Discussed during meeting: We think this is generally a good idea but specifics need to be worked out. Right now, our plan is to ask some distro (perhaps debian) for their configuration and test that stage0 bootstrap works with it. cc @alexcrichton

[est31]

One can apparently turn off network access via unshare: rust-lang/cargo#4770 (comment)

[Mark-Simulacrum]

We discussed this some more today and generally decided that we don't believe this is too viable to do today and is likely not too beneficial in general. We may revisit this in a more targeted manner later down the line (after CI migration concludes).