ci: sync cargo-check-external-types nightly

Author: cpu

.github/workflows/ci.yml

@@ -93,7 +93,7 @@ jobs:
             - name: Install rust toolchain
               uses: dtolnay/rust-toolchain@master
               with:
-                  toolchain: nightly-2025-10-18
+                  toolchain: nightly-2026-03-20
                   # ^ 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 for rcgen/