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.
2 jobs for wip/mcrha/update-notifications in 4 minutes and 38 seconds
Status Name Job ID Coverage
  Build
passed debian-stable-x86_64 #1189652

00:01:40

passed fedora-x86_64 #1189651

00:04:38