Add a flag for pretending to be a stable compiler when bisecting.

[oli-obk]

This should avoid having to patch ahash and similar crates to not do nightly feature detection and break the bisection (see https://github.com/rust-lang/rust/issues/123276#issuecomment-2075001510 for an example).

We could also do this by default and add an opt-out flag for when you want to bisect code that uses feature gates

[apiraino]

thank youuu :pray:

[lochetti]

Hey! Sorry to ask, but anyone know if we could merge/continue with this PR? :)

Thanks!

[ehuss]

@oli-obk Is this something you want to continue with? I forget what the status is.