krew release urls changed since v0.4.2, clearly OS type and arch inside the filename.

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

define `host_os` like `host_architecture` determine which OS is krew
installed at.