Introduce automation to keep `pnpm/action-setup` up-to-date

Hello! This issue was created semi-automatically because this repo popped up in a search. Per https://infra.apache.org/github-actions-policy.html, 3rd-party actions such as `pnpm/action-setup` must be referred to by hash and kept up-to-date by automation such as dependabot. It looks like this repo does not pollow this policy yet.

We plan to start enforcing this policy for `pnpm/action-setup` through https://github.com/apache/infrastructure-actions/pull/1193 .

If this was a false posivite, you can close this issue. Otherwise, you likely want to update your workflows, since they will stop working once that PR is merged.

Inspect the repository's GitHub Actions workflow files for uses of pnpm/action-setup, then read the Apache GitHub Actions policy linked in the issue. Update the workflows to meet the hash-pinning requirement and add the requested automation so that future versions remain current; done means the workflows comply and the automation covers pnpm/action-setup.

Written by the indexing model from the issue text.