We run builds 5 nights a week and upload them to SourceForge for public consumption. Both 4.12.0 and 5.0.0 pre-alphas have been there for a long time. If you're looking for a specific platform (e.g. Linux) and can't find it in the most recent nightly build, check one or two nights back.
Fair warning — download databases, Updater packages, and desktop client bundles from the same night. Bug fixes often require changes to both database and client application. Features almost always do. These changes may break extensions, which are not yet part of our nightly build process.
Note: There will be a lag between getting macOS, Linux, and Windows builds running when we switch Qt versions. This is infrequent but it does happen. Most recently we upgraded from Qt 5.5.1 to 5.11.2 and it took us a very long time to get the Windows builds running.