Add support so that Miri can automatically be quiet when the sysroot is unchanged

[saethlin]

Support for rust-lang/miri#3599

[RalfJung]

Looks good, thanks!
But could you please add doc comments to the new public items?

[saethlin]

I think I got it now. Also applied the lifetime change for rust-lang/miri#3599 (comment)

[RalfJung]

Ah, I didn't realize an extra lifetime parameter is needed to avoid pre-computing the message in Miri... is that worth it?

[saethlin]

🤷 I made it 'static initially because that's my usual lazy approach to programming. The lifetime param doesn't contaminate the API as far as I can tell, just adds some boilerplate to the implementation. And it makes calling easier.

[RalfJung]

doesn't contaminate the API

That's fair, the tests still work without even mentioning the lifetime.

However, if we ever get that lint against implicit lifetimes then the build_sysroot signature will need updating... but Miri and cargo-careful still don't as they do not ever give a type annotation to a SysrootBuilder, which is probably a rare thing to do for a builder anyway.

So, yeah, works for me. Thanks!