thumbnail: Fix non-working thumbnailer in Flatpak

In a Flatpak, $FLATPAK_SANDBOX_DIR might be set, but the directory might
not exist. This means that the thumbnailing sandbox might fail to start
because the shared directory is missing.

(test-desktop-thumbnail:6): GnomeDesktop-DEBUG: 16:37:21.007: Thumbnail script ('evince-thumbnailer -s %s %u %o') failed for 'file:///home/hadess/foo.cbz': Could not create temporary sandbox directory
1 job for wip/hadess/fix-thumbnailing-in-flatpaks in 5 minutes and 30 seconds
Status Name Job ID Coverage
passed build_stable #1170834