Downgrade nightly when adding component

[RalfJung]

Describe the problem you are trying to solve

I have a nightly installed and want to add a component that is not available for this or any newer nightly.

Describe the solution you'd like

With #2126 we have rustup toolchain install --allow-downgrade; this issue would be solved by adding the parallel rustup component add --allow-downgrade.

Notes

Currently available work-around: rustup toolchain install $already_installed_channel --allow-downgrade -c added_component_name

[kinnison]

Unfortunately @BeniCheni grew too busy to continue work on this issue, thus it is open for new contributors once more. Information on one possible approach (though Ben decided he didn't like it in the end) is in #2176 and support is available on Discord in the #wg-rustup channel.

[kinnison]

This turns out to be moderately hard given where the auto-downgrade logic is currently. I don't want to block a 1.22 release on this.

[kellda]

Currently available work-around: rustup toolchain install $already_installed_channel --allow-downgrade -c added_component_name

What about calling install_from_dist (the same function as rustup toolchain install calls) when --allow-downgrade is passed to rustup component add ? Would we need to prevent upgrading the toolchain in this case ?

[kinnison]

It would indeed need to prevent upgrade, which would complicate the install-from-dist interface again :(