Beta releases of apps

https://github.com/helsinki-systems/nc4nix/issues/4 removed all beta releases, but in some cases this is unwanted because you want to use the beta version. For example the News app only has beta releases for the most recent Nextcloud versions because the migration to Vue is not entirely done yet. This means I can't use nix to install the News app on these versions because there is no non-beta release available. On the other hand it makes sense that you normally only want non-beta releases, so both ways should be possible. How about providing both `<app>` and `<app>-beta` with the later also allowing beta releases? It won't conflict with any app names since dashes are not allowed in the names of apps. Maybe there is a better solution to this, or you just advise everyone to manually configure the desired release which would probably also be sensible.