CGO_ENABLED=0 in building linux binary

[mist714]

The current binary is dynamic-executable.

$ ldd kubectl-hns_linux_amd64 
	linux-vdso.so.1 (0x00007ffc3034b000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f608e2d7000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f608e50e000)

This is unavoidable for MacOS and Windows, but since Linux environments are required to operate under the minimum necessary conditions, I think we should make not a dynamic executable but a single-binary.
The concern is that the size will increase, but it is smaller than the original binary (62.2 MB -> 47 MB)

$ ls -hl bin/kubectl/kubectl-hns_linux_amd64
-rwxr-xr-x  1   staff    47M 11 14 17:04 bin/kubectl/kubectl-hns_linux_amd64*

[linux-foundation-easycla]

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• ✅ login: mist714 (4b37edd)

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[k8s-ci-robot]

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[rjbez17]

/ok-to-test
/lgtm

@adrianludwin any concerns here?

[rjbez17]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mist714, rjbez17

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