ci: update toolchain to nightly-2025-08-06 for cargo-check-external-types

Author: gcsbt

.github/workflows/rust.yml

@@ -90,7 +90,7 @@ jobs:
       - name: Install rust toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly-2024-05-01
+          toolchain: nightly-2025-08-06
           # ^ 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