[proptest-derive] warning about `non_upper_case_globals`

[NthPortal]

when using #[derive(Arbitrary)], the compiler warns that:

warning: allow(non_upper_case_globals) incompatible with previous forbid
 --> core/src/core/stack/index.rs:9:25
  |
9 | #[cfg_attr(test, derive(Arbitrary))]
  |                         ^^^^^^^^^ overruled by previous forbid
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
  = note: `forbid` lint level was set on command line
  = note: `#[warn(forbidden_lint_groups)]` (part of `#[warn(future_incompatible)]`) on by default
  = note: this warning originates in the derive macro `Arbitrary` (in Nightly builds, run with -Z macro-backtrace for more info)

I'd prefer to hard-forbid future_incompatible, especially since this is not technically part of my code.