Clear RUSTUP_* environment variables in test_env

[smoelius]

This PR clears all RUSTUP_* environment variables in effort to create a "clean" environment.

However, some tests fail when RUSTUP_HOME is cleared. This PR sets RUSTUP_HOME for those tests, so that they might be further investigated.

This PR came out of #16131 (comment) and #16131 (comment).

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[smoelius]

I could use some help determining why the doctests are failing.

[ehuss]

I reran the jobs. I think the runner was just flaky.

[smoelius]

I'm going to try to find machines resembling the failing platforms that I can test this on.

[epage]

So is the reason RUSTUP_HOME is needed for some tests is because the rustc that is in the path is the proxy and it needs to look up the actual rustc? Should we do that processing in cargo-test-support and prepend the PATH with the actual one?

[smoelius]

So is the reason RUSTUP_HOME is needed for some tests is because the rustc that is in the path is the proxy and it needs to look up the actual rustc? Should we do that processing in cargo-test-support and prepend the PATH with the actual one?

I'm still investigating. I will reply within the next few days.