lib/packagekit: Don't ever use PackageKit when /usr is mounted read-only
lib/packagekit: Don't ever use PackageKit when /usr is mounted read-only

PackageKit is sometimes installed on ostree systems, but that doesn't
mean we can install packages with it.

Closes #12562
moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,deryni/cockpit,garrett/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpituous/cockpit,cockpituous/cockpit,mvollmer/cockpit,garrett/cockpit,cockpituous/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,garrett/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpit-project/cockpit,deryni/cockpit,deryni/cockpit