Auto merge of #133068 - jieyouxu:download-rustc-default-only-for-tools, r=clubby789

Use `download-rustc=false` global default, `if-unchanged` for tools and library profiles, and make `rust.debug-assertions=true` inhibit downloading CI rustc

- Use `download-rustc = false` as global default.
    - Use `download-rustc = 'if-unchanged'` for tools and library profiles.
- Make `rust.debug-assertions = true` inhibit downloading CI rustc because alt rustc builds do not yet have rustc debug assertions enabled.

Fixes #133132.

cc discussions: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Bootstrap.20breakage
compiler contributors poll: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/.60download-rustc.20.3D.20'if-unchanged'.60.20for.20.60compiler.60.20profile.3F/near/481877253
library contributors poll: https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/.60download-rustc.20.3D.20.22if-unchanged.22.60.20default.20for.20libs.20profile.3F/near/482607011
cc https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/When.20is.20rustc.20built.20with.20debug.20assertions.3F

cc `@MarcoIeni` since you're working on improving CI job times, sorry, this will definitely regress some CI job times because we're probably lying to ourselves that CI rustc had debug assertions for some time 😅

cc `@onur-ozkan` for FYI, but since you're on vacation (sorry for the ping),

r? `@Kobzol` (I *think* you have a bit more context than other bootstrap reviewers?)

Author: bors