Build operator images more frequently to get security updates

[NickLarsenNZ]

Our operator images are currently only built when a pull request is merged.

RedHat fixed a lot of CVEs (e.g. in https://access.redhat.com/errata/RHSA-2024:3339) which disappeared in recently built operator images.

To avoid unnecessary work, we should rebuild the operator images regularly.

It should also be possible to trigger the build manually.

### Acceptance criterea
- [x] Run the build workflow on schedule, once per week (in the weekend).
- [x] Allow build workflow to be manually triggered