This change moves registry URL from registry.labs.nic.cz to
registry.nic.cz.

This fixes:

- WARNING: Failed to pull image with policy "always": Error response from daemon: Head https://registry.nic.cz/v2/turris/os/build/manifests/latest: unauthorized: HTTP Basic: Access denied (manager.go:205:0s)

-ERROR: Preparation failed: failed to pull image "registry.labs.nic.cz/turris/os/build" with specified policies [always]: Error response from daemon: Head https://registry.nic.cz/v2/turris/os/build/manifests/latest: unauthorized: HTTP Basic: Access denied (manager.go:205:0s)