thumbnailer: Cache GStreamer's plugin registry
This will speed up thumbnailers that need it.
This is done by sharing a directory that contains a GStreamer registry cache, distinct from the one that might have been generated by the Flatpak'ed application itself, and the directory is emptied on exit.
Another option might be to give it an empty directory (such as the one where we generate the thumbnail), and move the registry to a particular location after each run, and copy it back in the temp directory on each run.