chore: add mixins-preview build to service spec workflow (#36300)

### Issue # (if applicable)

N/A

### Reason for this change

See #36299 (comment)

### Description of changes

Added a step to build `mixins-preview` before the L1 generation step

### Describe any new or updated permissions being added

N/A


### Description of how you validated changes



### 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: vishaalmehrishi

.github/workflows/spec-update.yml

@@ -58,9 +58,9 @@ jobs:
       # Build @aws-cdk/spec2cdk and run L1 gen script to generate base files for new modules
       - name: Build @aws-cdk/spec2cdk
         run: lerna run build --stream --no-progress --skip-nx-cache --scope @aws-cdk/spec2cdk
-      - name: Generate L1s
-        working-directory: packages/aws-cdk-lib
+      - 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
       # Creating a pull request requires write permissions and it's best to keep write privileges isolated.