Provide SDK codegen unit tests with more space (#4328)

## Motivation and Context
The latest release workflow encountered no space left on device in
`check-sdk-codegen-unit-tests`:
```
       = note: /usr/bin/ld: final link failed: No space left on device
              collect2: error: ld returned 1 exit status
```

This PR updates its runner to `smithy_ubuntu-latest_8-core` to resolve
the issue.

----

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choice._

Author: ysaito1001

.github/workflows/ci.yml

@@ -99,7 +99,7 @@ jobs:
           runner: smithy_ubuntu-latest_8-core
           fetch-depth: 0
         - action: check-sdk-codegen-unit-tests
-          runner: ubuntu-latest
+          runner: smithy_ubuntu-latest_8-core
         - action: check-fuzzgen
           runner: ubuntu-latest
         - action: check-server-codegen-integration-tests