bots: Build rhel-x packages on rhel-x image
bots: Build rhel-x packages on rhel-x image

Stop building rhel-x packages on fedora-28. There is an ever-increasing
set of differences which we can't represent this way, and we were not
testing what really was built and shipped downstream.

Closes #9774

andreasn/cockpit,andreasn/cockpit,deryni/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,andreasn/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,andreasn/cockpit,cockpituous/cockpit,andreasn/cockpit,martinpitt/cockpit,deryni/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,andreasn/cockpit,andreasn/cockpit,cockpituous/cockpit,deryni/cockpit