feat: log the value(s) of `RUSTUP_UPDATE_ROOT`, `RUSTUP_DIST_SERVER` or `RUSTUP_DIST_ROOT` if set

[rami3l]

Closes #3680.

This is a tiny yet important feature addition for our users who need to manually designate a Rustup update/dist server, either because they have to use a mirror server for acceleration, or because they need to switch to the dev environment for beta testing.

[rami3l]

Note: The tests are failing as expected. I'll first use these tests to see whether the feature is working, and then add omission indicators to the UI tests if everything goes well.

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Oops, that's a whole lot of tests! This might need to wait for a bit longer since I'm a bit occupied with other stuff recently...