dh-exec: Strip the output even in the presence of --with-scripts

Even when the user explicitly sets the list of scripts, still do call
strip on the output.

Thanks to Dann Frazier <dannf@debian.org> and Daniel Schepler
<dschepler@gmail.com> for reporting the issue, in #711937 and #720418.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>