add new config: rust.download_rustc_incompatibility_check

[onur-ozkan]

Unlike #142054, this adds a config-based approach to control download-rustc incompatibility check.

Fixes #142054

[rustbot]

r? @jieyouxu

rustbot has assigned @jieyouxu.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

This PR modifies bootstrap.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[jieyouxu]

Waiting on discussions to settle on #t-infra/bootstrap > Settings are incompatible with download-rustc.

@rustbot label -S-waiting-on-review +S-waiting-on-team

[bors]

☔ The latest upstream changes (presumably #142070) made this pull request unmergeable. Please resolve the merge conflicts.

[onur-ozkan]

I will hold rebasing until we decide to merge it.

[onur-ozkan]

#t-infra/bootstrap > stepping down from T-Bootstrap