bots: Work around pbuilder build dep resolution bug in ubuntu-1604
bots: Work around pbuilder build dep resolution bug in ubuntu-1604

16.04's pbuilder-satisfydepends-classic does not correctly resolve our
alternative debhelper build dependency and does not install dh-systemd
(even though the available debhelper is older than 9.20160709). This is
not an issue in newer releases, so just manually install dh-systemd.

Closes #7875 
Reviewed-by: Stef Walter <8a7fbf0f15d3ad982af54ef28f5c117d2494b76c@redhat.com>
andreasn/cockpit,garrett/cockpit,andreasn/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,Scribery/cockpit,andreasn/cockpit,andreasn/cockpit,stefwalter/cockpit,martinpitt/cockpit,petervo/cockpit,Scribery/cockpit,cockpituous/cockpit,deryni/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,cockpituous/cockpit,mareklibra/cockpit,garrett/cockpit,petervo/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,petervo/cockpit,stefwalter/cockpit,stefwalter/cockpit,deryni/cockpit,mvollmer/cockpit,Scribery/cockpit,petervo/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,Scribery/cockpit,deryni/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,deryni/cockpit,andreasn/cockpit,cockpit-project/cockpit,petervo/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,Scribery/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mareklibra/cockpit,petervo/cockpit,petervo/cockpit,martinpitt/cockpit,Scribery/cockpit,garrett/cockpit,stefwalter/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,mvollmer/cockpit,stefwalter/cockpit,mareklibra/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,garrett/cockpit,mvollmer/cockpit,andreasn/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,Scribery/cockpit,stefwalter/cockpit,cockpituous/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,deryni/cockpit,mareklibra/cockpit