krew: update to v0.4.2 (#8168)

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.

roles/kubespray-defaults/defaults/main.yaml

@@ -576,6 +576,17 @@ host_architecture: >-
   {{ ansible_architecture }}
   {%- endif -%}
 
+_host_os_groups:
+  Linux: linux
+  Darwin: darwin
+  Win32NT: windows
+host_os: >-
+  {%- if ansible_system in _host_os_groups -%}
+  {{ _host_os_groups[ansible_system] }}
+  {%- else -%}
+  {{ ansible_system }}
+  {%- endif -%}
+
 # Sets the eventRecordQPS parameter in kubelet-config.yaml. The default value is 5 (see types.go)
 # Setting it to 0 allows unlimited requests per second.
 kubelet_event_record_qps: 5