chore: Pin nightly version for check-external-types

This is is because the versions are extremely tied to each other.

Author: hdost

.github/workflows/ci.yml

@@ -59,7 +59,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: nightly-2023-10-10
           components: rustfmt
           override: true
       - name: external-type-check