Fix quotation of nerdctl_extra_flags (#8668)

Due to missing quotation of nerdctl_extra_flags, ansible-playbook was failed:

  Using module file /usr/local/lib/python3.6/dist-packages/ansible/modules/command.py
  Pipelining is enabled.
    [..]
    File "/usr/lib/python3.8/shlex.py", line 191, in read_token
      raise ValueError("No closing quotation")

This fixes the issue.

T-Eberle investigated the issue and found the solution.
Thank you T-Eberle!