Don't fail "ostree remote refs" if writing the summary cache is not permitted

Don't fail "ostree remote refs" if writing the summary cache is not permitted

It used to be allowed to run something like "ostree remote refs" on
a read-only (e.g. system) repo. However, the summary cache caused that to
break. This commit just makes it not save the cache if we get some kind
of permission error when writing it. It'll still work, even without the
cache.

https://bugzilla.gnome.org/show_bug.cgi?id=763855