run: tweak how we name our transient units

run: tweak how we name our transient units

The current logic is a bit complex how systemd-run units are called. It
used to be just the unique ID of the dbus connection. Which was nice,
since its system-widely, uniquely assigned to us. But this didn't work
out well, due to direct connections to PID 1 and due to soft reboots.

We nowadays have a better ID to use though, with nicer properties: the
kernel manages a pidfd ID for every process after all, and it's globally
unique, for any process, and regardless of soft reboots. Hence use that
for naming preferably, and just keep one branch with a randomized name
as fallback.