Update cargo-check-external-types rustc version

[cpu]

:wave: As promised, when cargo-check-external-types makes an update I'm opening PRs to fix the builds :-)

Note: there seems to be another unrelated breakage with nightly clippy, a finding from unexpected-cfgs. I'm not sure what the right fix is in this case: it seems like "exact_size_is_empty" is being used as a feature gate for the unstable nightly feature (https://github.com/rust-lang/rust/issues/35428) and I'm not sure how to square that requirement with the new lint. Suggestions welcome!

[chifflier]

Hi, I checked but it seems that there is no easy way for us to check for the nightly feature. Anyway, the function was mostly provided as a helper, we do not use it. Removed in 382dacf

[chifflier]

Applied, thanks!