krew: update to v0.4.2

[zhengtianbao]

krew release download urls changed since v0.4.2.

from:
https://github.com/kubernetes-sigs/krew/releases/download/v0.4.1/krew.tar.gz
to:
https://github.com/kubernetes-sigs/krew/releases/download/v0.4.2/krew-linux_amd64.tar.gz

Release separated archives for each platform, so we should define host_os like host_architecture to determine which OS krew will be installed at.

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR upgrades krew to 0.4.2 and support darwin/windows platform.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

krew: upgrade to v0.4.2

[k8s-ci-robot]

Hi @zhengtianbao. Thanks for your PR.

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[floryut]

@zhengtianbao 👍 nicely done
/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, zhengtianbao

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[cristicalin]

Nice work @zhengtianbao , thank you!

/lgtm