Regular (e.g. weekly) job to verify build against GitHub Actions environment

[rolyp]

GitHub Actions regularly (every few months) drops support for specific Python versions. There are likely other unstable aspects of the build environment. Given this, a badging saying "passing" for a build that hasn't run for months is rather meaningless. Add a regular job (e.g. a weekly build) that sanity-checks the build against the current environment. This will definitely fail at some point after the project is no longer under active development, but this seems preferable to pretending that it builds correctly, even though a build may not have run for weeks or months.