make update-codegen.sh runnable on Mac

[spacewander]

What type of PR is this?

/kind bug

What this PR does / why we need it:

uname -o isn't supported on Mac. Here is the output of man uname:

UNAME(1)                                                           General Commands Manual                                                          UNAME(1)

NAME
     uname – Print operating system name

SYNOPSIS
     uname [-amnprsv]

DESCRIPTION
     The uname utility writes symbols representing one or more system characteristics to the standard output.

     The following options are available:

     -a      Behave as though all of the options -mnrsv were specified.

     -m      print the machine hardware name.

     -n      print the nodename (the nodename may be a name that the system is known by to a communications network).

     -p      print the machine processor architecture name.

     -r      print the operating system release.

     -s      print the operating system name.

     -v      print the operating system version.

     If no options are specified, uname prints the operating system name as if the -s option had been specified.

SEE ALSO
     hostname(1), machine(1), sw_vers(1), uname(3)

STANDARDS
     The uname utility conforms to IEEE Std 1003.2-1992 (“POSIX.2”).  The -p option is an extension to the standard.

macOS 12.6
November 9, 1998
macOS 12.6

Also fix a passthrough bug when the script is running on Unsupported OS.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

Hi @spacewander. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[youngnick]

SGTM

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: spacewander, youngnick

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[youngnick]

/ok-to-test

[robscott]

Thanks @spacewander!

/lgtm