VMware vCSA 6.0 Update 1 nobody's password

When trying to update a VMware vCenter Server Appliance to 6.0 Update 1 via SSH, I ran into an authentication problem.

I logged into a VMware vCenter Server Appliance via SSH and tried to update it to 6.0 Update 1 by entering the following command:

Command> software-packages install --iso --acceptEulas

I was then requested to enter a password for nobody

Command> software-packages install --iso --acceptEulas
nobody's password:

The problem was that I logged in with a VMware SSO account, Administrator@vsphere.local, and actually needed to login with the vCSA's root account.

What is the nobody account?
On Unix based systems like Linux, it is an account with limited permissions.