Lock tokio version in MSRV CI tests

Starting with v1.39, tokio bumped its MSRV to Rust 1.70. This causes the
MSRV pipeline to fail unless we manually lock the tokio version.

.github/workflows/ci.yml

@@ -81,8 +81,10 @@ jobs:
         run: rustup default 1.66
       - uses: Swatinem/rust-cache@v2
 
+      - name: Lock dependencies for MSRV
+        run: cargo update --precise 1.38.1 tokio
       - name: Run unit tests
-        run: cargo test --all-features -- --color=always
+        run: cargo test --locked --all-features -- --color=always
 
       - name: Run end-to-end tests
         if: inputs.test-all || github.event_name == 'schedule'