emit feature help in cheat mode (fix nightlies)
This should fix the `distcheck` failure in the latest nightly.
cc #36539
It's probably not ideal to check the environment that often and the code ist duplicated from `librustc/session/config.rs` but this was the easiest fix I could think of.
A cleaner solution would probably be to move the `unstable_features` from `Options` to `ParseSess` and change the `diag` parameter of `emit_feature_err` to take `ParseSess` instead of a `Handler`.
d0623cf7bd