ci: Update to cargo-check-external-types 0.4.0 (#633)

Author: tottoto

.github/workflows/CI.yml

@@ -101,11 +101,11 @@ jobs:
     - uses: actions/checkout@v6
     - uses: dtolnay/rust-toolchain@master
       with:
-        toolchain: nightly-2025-08-06
+        toolchain: nightly-2025-10-18
     - name: Install cargo-check-external-types
       uses: taiki-e/cache-cargo-install-action@v2
       with:
-        tool: cargo-check-external-types@0.3.0
+        tool: cargo-check-external-types@0.4.0
     - uses: Swatinem/rust-cache@v2
     - run: cargo check-external-types --all-features
       working-directory: tower-http