Fixes <https://bugs.gnu.org/43720>.
Reported by Andreas Enge <andreas@enge.fr>.
The bug was introduced in 5c793753b31b1dcd9a554bce953124f7ae88ca9a,
which changed the way TO-START is computed: as a function of the running
services first, and then as a function of the live services (which
includes services not currently running).
* guix/scripts/system/reconfigure.scm (running-services): Serialize the
'running' field and return it.
(upgrade-shepherd-services): Comput RUNNING. Compute TO-START as the
difference between TARGET-SERVICES and RUNNING.
