ci: fix clippy finding, update cargo-check-external-types toolchain

[cpu]

👋 As promised, when cargo-check-external-types makes an update I'm opening PRs to fix the builds :-) This project also had a clippy finding that is fixed along the way.

Cargo: add missing x500 feature

This was being ref'd in oid_db.rs, producing a clippy error of the form:

error: unexpected `cfg` condition value: `x500`
   --> /home/runner/work/oid-registry/oid-registry/target/debug/build/oid-registry-fa6fec30f9e743c7/out/oid_db.rs:411:11
    |
411 |     #[cfg(feature = "x500")]
    |           ^^^^^^^^^^------
    |                     |
    |                     help: there is a expected value with a similar name: `"x509"`
    |
    = note: expected values for `feature` are: `crypto`, `default`, `kdf`, `ms_spc`, `nist_algs`, `pkcs1`, `pkcs12`, `pkcs7`, `pkcs9`, `registry`, `x509`, and `x962`

The solution seems simple: add the missing x500 feature.

ci: update cargo-check-external-types toolchain

The upstream project cut a 0.1.12 release that now pins Rust nightly-2024-05-01. This commit updates CI to match.

[chifflier]

Applied, thanks!