Private Flatpak installations in Builder

Builder needs to deal with many SDK and SDK extensions for applications built upon Flatpak.

One thing I never liked about how we did this up until now was that we needed to install Flatpak remotes into the user’s personal Flatpak installation. First, because we needed to add Flathub and gnome-nightly repositories. Secondly, once a year we need to add the flathub-beta remote due to post-branch SDKs relying on beta extensions.

Previously this would pollute things like GNOME Software with versions of applications that you might not care about as a user.

In Builder 41, a private Flatpak installation is used in $XDG_DATA_DIRS which contains those remotes. Additionally we set a filter to only allow runtimes and specifically ones matching certain globs.