Update URL of gpg key to use for kubectl install

[AlanGreene]

Changes

Google are now serving the key in ascii armored format instead of the previous binary format and as a result has broken the E2E docker image build. To avoid installing additional tools such as gpg, switch to the backup k8s URL which was implemented in response to previous issues with the Google-hosted key being unavailable.

/kind misc

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[briangleeson]

/lgtm

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