pin grcov version

[jprendes]

Recently CI has been failing due to an update to grcov, Foe example, here

Run grcov . \
Error: 3 [ERROR] A panic occurred at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/grcov-0.9.0/src/llvm_tools.rs:95: called `Result::unwrap()` on an `Err` value: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }
Error: 3 [ERROR] A panic occurred at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/grcov-0.9.0/src/llvm_tools.rs:95: called `Result::unwrap()` on an `Err` value: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }
Error: 3 [ERROR] A panic occurred at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ignore-0.4.23/src/walk.rs:[13](https://github.com/containerd/rust-extensions/actions/runs/14349512282/job/40225446691?pr=389#step:6:14)01: called `Result::unwrap()` on an `Err` value: Any { .. }
Error: 3 [ERROR] A panic occurred at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/grcov-0.9.0/src/llvm_tools.rs:95: called `Result::unwrap()` on an `Err` value: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }
Error: Process completed with exit code 1.

This PR pins the grcov version to unblock CI.

[jprendes]

@mxpv PTAL, thanks! :-)

[mxpv]

Thanks!