publish CI maintenance: Update Python, use build, run on PRs

A bit of maintenance on the publish CI workflow:

Run also on Pull Requests and when manually triggered (workflow_dispatch)
Use the latest stable checkout version (v3)
Use the latest stable Python version (3.x)
Switch to Build for distribution building, since calling setup.py directly is deprecated (see Why you shouldn't invoke setup.py directly)
- Only publish when the workflow is triggered by a push and the commit is a tag

coin-or / pulp