Warn when installing with a non-default toolchain

[smoelius]

This PR implements the cargo changes for #11036. The rustup changes were implemented in rust-lang/rustup#4518.

The PR is currently organized as four commits:

• The first expands the rustup tests to set RUSTUP_TOOLCHAIN_SOURCE. This change is not strictly necessary, but it improves the rustup tests' fidelity.
• The second moves the pkg function to a location where the next commit can use it.
• The third implements a test to check the fourth commit's fix.
• The fourth warns when cargo install is invoked with a non-default toolchain, as indicated by RUSTUP_TOOLCHAIN_SOURCE.

In the third commit, two significant changes were made to simulated_rustup_environment:

• Previously, the constructed proxy would call an always-panicking executable whenever it was asked to run cargo. Now, the proxy can be made to run the cargo under test.
• The proxy can now be made to set additional environment variables (e.g., RUSTUP_TOOLCHAIN_SOURCE) before calling the proxied tool.

The PR is currently marked as draft because rust-lang/rustup#4518 has not yet been released. Technically, this PR could be merged before then. But testing it with a fixed rustup would seem to make sense.

Nits are welcome.

cc: @epage

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[weihanglo]

EDIT: The force push was to make Clippy happy.

Feel free to force-push and re-organize your commits during the iterations.

[smoelius]

Just FYI, I know it looks like only some of the comments have been addressed. I am still hoping to get feedback on #16131 (comment) and #16131 (comment).

[smoelius]

I resolved comments that I thought I implemented uncontroversially. I hope that was okay.

EDIT: I also hid a few of my own comments that were old.

[epage]

Please clean up your commits for how they should be reviewed and merged, rather than for how it was developed.

[smoelius]

I need to fix the commits. I will do that within the next few hours.

[smoelius]

The commits should be fixed now.

[epage]

Thanks for your patience through the rounds of feedback! This is looking great!

[epage]

@rami3l when would we be good for merging support for this?

[rami3l]

@epage I'd say after v1.29.0 has been properly released and stabilized for a bit, but that will be pretty close from here. Judging from the current progress, I'd say the release could happen next week or another week after that, unless new blocking PRs resurface by then. I'm planning that out depending on my availabilities during the night. Please stay tuned and many thanks in advance for your comprehension!