pull: Don't save summary to cache before validating signatures

pull: Don't save summary to cache before validating signatures

In case of some kind of race or other weirdness we might be getting
non-matching versions of summary.sig and summary, where summary.sig
is the latest version. Currently we're saving them to the cache
directly after downloading them successfully, but they will then fail
to gpg validate. Then on the next run we'll keep using the cached files
even if they are incorrect, until summary.sig changes upstream.

This changes the order so that we verify the signatures before saving
to the cache, thus ensuring that we don't end up in a stuck state.

Fixes https://github.com/ostreedev/ostree/issues/1523

Closes: #1529
Approved by: cgwalters