A way to test differences in diagnostics between stable and nightly

[Centril]

Sometimes we do things like if self.sess.opts.unstable_features.is_nightly_build() { to switch between diagnostics for stable and nightly, e.g. as in #65893.

It would be good to have a way to tell a nightly compiler to behave as-if it was a stable one by making .is_nightly_build() return false. Ideally, this would be provided as a -Z flag which can be used in UI tests.

cc @estebank

[fmease]

Since #132993 setting the env var RUSTC_BOOTSTRAP to -1 achieves this.