core: allow specifiers to be resolved in Environment= passed over dbus (#6144)

core: allow specifiers to be resolved in Environment= passed over dbus (#6144)

When specifiers are included in the Environment block in StartTransientUnit,
we resolve specifiers on the PID1 side. Nevertheless we store the unresolved
version in the transient unit file, so that it'll be resolved when loading
the unit. I think this looks nicer.

I also removed the writing of the merged Environment block to the transient
file. Afaict, this resulted in variables being written multiple times, but
this needs to be tested properly.

Fixes #5699.