Pin Rust version in CI

Author: bikeshedder

.github/workflows/rust.yml

@@ -39,6 +39,7 @@ jobs:
           --health-timeout 5s
           --health-retries 5
     steps:
+      - run: rustup update 1.85 && rustup default 1.85
       - uses: actions/checkout@v4
       - name: Build
         run: cargo build --all-features --workspace