Rebuild stable release more often

Currently we only build and export the stable branch when we do a release, however, that means that we don't get the bug fixes that happens upstream in GNOME in our current stable. A example of that is elementary/browser#52 that got fixed in WebKitGtk 2.40.1, but our last build of the 7.2 branch was before that, and thus, include only WebKitGtk 2.40.0 (for a matter of comparison, the daily current includes WebKitGtk 2.40.2).

We should do rebuilds of our current stable version when changes in the GNOME runtime occurs, Maybe we can do something with the `--rebuild-on-sdk-change` and `--require-changes` options of flatpak-builder.