add pull_request trigger to test_zcu111.yml

[meeg]

We're going to enable automatic testing - limited so far to @ammounce's ZCU111 and the first demo notebook, but we will add more tests.

Workflow runs for PRs from outside contributors will always need to be approved by admins, some references for the security issues:

• https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#controlling-changes-from-forks-to-workflows-in-public-repositories
• https://github.com/orgs/community/discussions/26722#discussioncomment-3253085
• https://github.com/actions/runner/issues/494#issuecomment-887560662
• https://adnanthekhan.com/2023/12/20/one-supply-chain-attack-to-rule-them-all/