bots: Fix parallel image-customize runs
bots: Fix parallel image-customize runs

Don't hardcode the label to "install", as that prevents running
image-customize multiple times in parallel. Just let `TestVM` figure out
an unique label by itself.

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