ci: pin flate2 and once_cell in msrv job (#549)

Author: seanmonstar

.github/workflows/CI.yml

@@ -79,6 +79,8 @@ jobs:
         tool: protoc@3.20.3
     - run: cargo update -p tokio --precise 1.38.1
     - run: cargo update -p tokio-util --precise 0.7.11
+    - run: cargo update -p flate2 --precise 1.0.35
+    - run: cargo update -p once_cell --precise 1.20.3
     - run: cargo test -p tower-http --all-features
 
   style: