build: don't start example instance in postinstall

After tarantool installation on Debian/Ubuntu from repo, example
instance was automatically started on 3301 port. At the same time
example instance on RHEL/CentOS is started manually. Patch does the same
for Debian/Ubuntu.

Closes #4507

Reviewed-by: Igor Munkin <imun@tarantool.org>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>