build: Run build workflow on PRs.

[nzdjb]

It would be useful to have the build job run on PRs.

Some notes on this:

• No secrets are passed to workflows run from forks, except for the GITHUB_TOKEN, which is provided with read-only privileges. This prevents secret leakage. (https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflows-in-forked-repositories-1)
• A maintainer with write permission will need to approve runs from first-time contributors. (https://docs.github.com/en/actions/managing-workflow-runs/approving-workflow-runs-from-public-forks)