sysupdate: Emit READY=1 status when installing

sysupdate: Emit READY=1 status when installing

`READY=1` is already correctly emitted when acquiring an update, but was
forgotten to be emitted when subsequently installing that update.

That meant that the state tracking code in `sysupdated` and hence
`updatectl` could not properly report the progress of the install
operation, and hence printed “Already up-to-date” after a successful
update installation, rather than “Done”.

Add a test to catch this in future.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Fixes: https://github.com/systemd/systemd/issues/41502