Handling of broken nightlies is bad #5
Description
Completely broken nightlies don't exist, only those where -Zbuild-std is broken, but that is surfaced as every target failing.
So the is_broken field on finished_nightly is kind of useless and should never be true, only for transient installation errors.
It should be removed with more graceful recovery instead.
RUSTUP_HOME should be better isolated anyways to allow for concurrent installations.