Update the runner type for check-client-codegen-unit-tests (#4201)

## Motivation and Context To fix the [No space left on device](https://github.com/smithy-lang/smithy-rs/actions/runs/16148239040/job/45581865654#step:4:440) error encountered during the release. ---- _By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice._

Author: ysaito1001

.github/workflows/ci.yml

@@ -97,7 +97,7 @@ jobs:
  - action: check-client-codegen-integration-tests
  runner: smithy_ubuntu-latest_8-core
  - action: check-client-codegen-unit-tests
- runner: ubuntu-latest
+ runner: smithy_ubuntu-latest_8-core
  - action: check-core-codegen-unit-tests
  runner: smithy_ubuntu-latest_8-core
  - action: check-rust-runtimes