Removed quotation of nerdctl_extra_flags. (#8695)

The quotations in the variable nerdctl_extra_flags are not required for the `nerdctl_image_pull_command` and throw the following error when executing the cluster-playbook with `container_insecure_registries` set:
        unknown flag: --insecure-registry\\\"
This happens as the complete nerdctl_image_pull_command string variable gets split into an array string for the cmd task. The escaped quotation doesn't get escaped properly and is added to the cmd-string array as part of the command. This leads to a wrong written insecure-registry flag, which throws this error.