Release archive contain additional files

I'm in the process of automating the Krew install on our workstations and found that at least the latest Krew release archive (v0.4.5) contains some MacOS specific files that IMHO should not be there. I haven't checked the archives for older versions yet.

If it's appreciated, I could make some changes to https://github.com/kubernetes-sigs/krew/blob/master/hack/make-release-artifacts.sh and create a pull request, so that cleaner archives are created in the future.