lib/repo-finder-mount: Fix path to flatpak repo

lib/repo-finder-mount: Fix path to flatpak repo

OstreeRepoFinderMount checks mounts for a few well-known directories
such as "ostree/repo" and ".ostree/repo" to try to find remotes. One of
the hard-coded directories is "var/lib/flatpak" but that's the flatpak
directory, not the ostree repo used by flatpak, which is at
"var/lib/flatpak/repo". So this commit changes the path so the repo can
be found.

For recent versions of Endless, flatpak uses /ostree/repo as its
repository, so this commit won't make a difference there. But it may
help on other operating systems.

Closes: #1471
Approved by: cgwalters