Use a SHA for the dtolnay/rust-toolchain action

Author: martin-g

.github/workflows/deploy-docs.yml

@@ -203,7 +203,7 @@ jobs:
           repository: 'apache/avro-rs'
 
       - name: Rust Toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable
         with:
           toolchain: stable