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 workflow files for references to `pnpm/action-setup`, then compare them with the Apache GitHub Actions policy linked in the issue. Done means the action is referred to by hash and automation such as Dependabot is configured to keep it up to date; close the issue if the report is a false positive.

Written by the indexing model from the issue text.