Remove --kube-root deprecated kind argument

[stmcginnis]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

The --kube-root flag has been deprecated in kind for some time and will be removed. The current equivalent functionality is to pass the path to the kubernetes source as the first non-flag argument to build node-image.

This updates the prow script to use the newer preferred method in preparation for kind dropping the flag.

Which issue(s) this PR fixes:

Related: https://github.com/kubernetes-sigs/kind/issues/3717

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

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