chore: set pr author to aws-cdk-automation (#20267)

The `peter-evans/create-pull-request` action sets the author of the pull
request to
`${{ github.actor }} <${{ github.actor }}@users.noreply.github.com>`
by default. In a scheduled workflow, the `actor` is the user who created
the workflow initially which is `nija-at` (which is not what we want).


----

### All Submissions:

* [ ] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

.github/workflows/yarn-upgrade.yml

@@ -120,6 +120,7 @@ jobs:
         with:
           # Git commit details
           branch: automation/yarn-upgrade
+          author: aws-cdk-automation <aws-cdk-automation@users.noreply.github.com>
           commit-message: |-
             chore: npm-check-updates && yarn upgrade
             Ran npm-check-updates and yarn upgrade to keep the `yarn.lock` file up-to-date.