Update GH workflows tests, adding pull request

[johnbaldwin]

Before this commit, a PR on figures from a fork will not trigger the Tox tests. This PR does the following

• Only run 'on push' for specified branches (main and develop/maple)
• Trigger 'on pull_request' (for all PRs)

The update here matches that of other repos we have, such as edx-platform, edx-theme-customers, course-access-group

[johnbaldwin]

Thank you both!