test/common/run-tests: clarify env section
test/common/run-tests: clarify env section

Closes #14217

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