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.

Author: cpu

.github/workflows/rust.yml

@@ -90,7 +90,7 @@ jobs:
       - name: Install rust toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly-2024-02-07
+          toolchain: nightly-2024-05-01
           # ^ sync with https://github.com/awslabs/cargo-check-external-types/blob/main/rust-toolchain.toml
       - run: cargo install cargo-check-external-types
       - run: cargo check-external-types