run: add --job-mode= argument (#34708)

systemctl has a --job-mode= argument, and adding the same argument to
systemd-run is useful for starting transient scopes with dependencies.
For example, if a transient scope BindsTo a service that is stopping,
specifying --job-mode=replace will wait for the service to stop before
starting it again, while the default job mode of "fail" will cause the
systemd-run invocation to fail.

diff --cc src/core/job.c
Simple merge

diff --cc test/units/TEST-74-AUX-UTILS.run.sh
Simple merge