bots: Fix TOCTOU race in machine network setup
bots: Fix TOCTOU race in machine network setup

With parallel tests, this would sometimes crash with

    FileExistsError: [Errno 17] File exists: '/tmp/.cockpit-test-resources'

Closes #11867
cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,mvollmer/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,mvollmer/cockpit,deryni/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,garrett/cockpit,garrett/cockpit,garrett/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit