run: expand variables also with --scope

run: expand variables also with --scope

This makes syntax be the same for commands which are started by the manager and
those which are spawned directly (when --scope is used).

Before:
$ systemd-run -q -t echo '$TERM'
xterm-256color

$ systemd-run -q --scope echo '$TERM'
$TERM

Now:
$ systemd-run -q --scope echo '$TERM'
xterm-256color

Previous behaviour can be restored via --expand-environment=no:
$ systemd-run -q --scope --expand-environment=no echo '$TERM'
$TERM

Fixes #22948.

At some level, this is a compat break. Fortunately --scope is not very widely
used, so I think we can get away with this. Having different syntax depending
on whether --scope was used or not was bad UX.

A NEWS entry will be required.