copy-worker: Run later, but before gnome-keyring-daemon
In gnome-build-meta#821 (closed) we saw that running so early, with DefaultDependencies=no, causes issues when GVFS (via GFile) tries to use D-Bus.
!249 (merged) configured GIO to use only the local VFS, but still, there is no need to run quite so early: the key thing is to copy the user's keyring before the keyring daemon.
Adjust the unit to attach to default.target (not basic.target), reinstate default dependencies, and explicitly run Before gnome-keyring-daemon.service.
See !249 (merged) for further discussion.