zkvm: fix run_tests `zkvm` is single-threaded, similar to `emscripten` and `wasm`. The `cfg` for `zkvm` seems to have been dropped. This PR adds the `cfg` again.
zkvm: fix run_tests `zkvm` is single-threaded, similar to `emscripten` and `wasm`. The `cfg` for `zkvm` seems to have been dropped. This PR adds the `cfg` again.