Sanitize environment variables for build scripts

[matklad]

Build scripts have access to the parent environment, and this may cause problem because it's not obvious which env vars are actually used by the build script, which hurts reproducible builds.

Perhaps we should use some kind of a whitelist of envvars to pass to build scripts?

[matklad]

cc @jsgf (https://internals.rust-lang.org/t/rfc-implement-a-sandbox-for-environment-variables-and-files/7213)

[jsgf]

https://github.com/jsgf/rfcs/blob/env-sandbox/text/0000-env-sandboxing.md

[matklad]

Discussed during a meeting: this might be a nice idea in theory, but it really needs some significant design work, to make sure this feature is actually usable.

[kornelski]

The "non-obvious" part doesn't have to be solved by blocking env vars, but can be solved by tracking which ones were used.

This is already almost a solved problem, but it's blocked on a showstopping bug: #4587