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.

----

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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