Bug 27844: Launch koha-worker systemd service as koha linux user

Bug 27844: Launch koha-worker systemd service as koha linux user

This patch adds the User directive to the koha-worker systemd unit file

Test plan:
0a. DO NOT APPLY PATCH YET
0b. Start a Koha testing environment running systemd
1. koha-worker --stop kohadev
2. cp debian/templates/koha-worker@.service /etc/systemd/system/.
3. systemctl start koha-worker@kohadev.service
4. ps -efww | grep "background_jobs_worker"
5. Note koha-worker is running as root
6. APPLY THE PATCH
7. cp debian/templates/koha-worker@.service /etc/systemd/system/.
8. systemctl daemon-reload
9. systemctl restart koha-worker@kohadev.service
10. ps -efww | grep "background_jobs_worker"
11. Note that koha-worker is now running as kohadev-koha

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>