gs-plugin-loader: Reset online-updates-timestamp only when did the update
The gs_plugin_loader_generic_update() is used in two cases, when updating the software and when downloading the software. The former case can reset the online-updates-timestamp, but the later not, because it did not update anything.
This blocks the update notifications from showing up after the first run of the Software.
This had been found during the Fedora tests here:
https://bugzilla.redhat.com/show_bug.cgi?id=1930401