ci: take updated nightly for cargo-check-external-types

Author: cpu

.github/workflows/rust.yml

@@ -100,7 +100,7 @@ jobs:
       - name: Install rust toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly-2025-05-04
+          toolchain: nightly-2025-08-06
           # ^ sync with https://github.com/awslabs/cargo-check-external-types/blob/main/rust-toolchain.toml
       - run: cargo install --locked cargo-check-external-types
       - name: run cargo-check-external-types