Leapp fails to upgrade the system in reboot phase and prints "Message: Failed to append extra arguments to kernel command line."

Issue

• Upgrading a RHEL 7 system to RHEL 8 fails in the reboot phase, drops to emergency mode and prints the following error message:

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                             ERRORS
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  2021-06-05 15:20:04.141285 [ERROR] Actor: kernelcmdlineconfig
  Message: Failed to append extra arguments to kernel command line.
  Summary:
      Details: Command ['grubby', '--update-kernel=/boot/vmlinuz-4.18.0-305.3.1.el8_4.x86_64', '--args=net.ifnames=0'] failed with exit code 1.
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                         END OF ERRORS
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