Add OS X sed check to update-submodule.sh

[roycaihw]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Add the same sed check in update-client.sh to update-submodule.sh, because update-submodule.sh also contains a sed -i command:

sed: 1: "/tmp/python-base-relnot ...": extra characters at the end of p command

Does this PR introduce a user-facing change?

NONE

/cc @Yashks1994 @yliaog

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In response to this:

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Add the same sed check in update-client.sh to update-submodule.sh, because update-submodule.sh also contains a sed -i command:

sed: 1: "/tmp/python-base-relnot ...": extra characters at the end of p command

Does this PR introduce a user-facing change?

NONE

/cc @Yashks1994 @yliaog

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[k8s-ci-robot]

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[yliaog]

better to refactor it out into a util later

/lgtm