fix: ci fix for spec updater workflow (#36364)

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Closes #<issue number here>.

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### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: kumvprat

.github/workflows/spec-update.yml

@@ -59,7 +59,6 @@ jobs:
       - name: Build @aws-cdk/spec2cdk
         run: lerna run build --stream --no-progress --skip-nx-cache --scope @aws-cdk/spec2cdk
       - name: Generate code
-        run: yarn gen
         run: lerna run gen --stream --no-progress --skip-nx-cache --scope aws-cdk-lib --scope @aws-cdk/mixins-preview
 
       # Next, create and upload the changes as a patch file. This will later be downloaded to create a pull request