Add cronjob to trigger nightly release builds

[troglobit]

The release workflow only runs when we tag a release. To avoid odd test failures at release time we therefore have so far had regression testing disabled in the release workflow.

After today's CCB it was decided to change this, adding the regression tests to the release workflow and, in an effort to locate any "flaky" tests, automatically run the release workflow every night, a sort of nightly job, which uploads its artifacts to the latest tag.

Also, remember to make it possible to run these test release jobs without any caches, e.g., ccache, so we can catch any such regressions as well.