Use pinned PR version of external types

Author: aumetra

.github/workflows/ci.yml

@@ -77,7 +77,7 @@ jobs:
           components: rustfmt
       - name: external-type-check
         run: |
-          cargo install cargo-check-external-types
+          cargo install cargo-check-external-types --git https://github.com/awslabs/cargo-check-external-types.git --rev 98d6e533ab92540ab12a4ca35acd244dacb04a9f
           cd ${{ matrix.example }}
           cargo check-external-types --config allowed-external-types.toml
   non-default-examples: