kn fails to build for Windows platform

[navidshaikh]

Current master fails to build for Windows

 GO111MODULE=on GOOS=windows GOARCH=amd64 go build -mod=vendor ./cmd/...
# github.com/knative/client/pkg/kn/commands/plugin
pkg/kn/commands/plugin/plugin_verifier.go:138:11: undefined: syscall.Stat_t
pkg/kn/commands/plugin/plugin_verifier.go:140:32: undefined: syscall.Stat_t

[navidshaikh]

We should delegate the executable check for plugin to underlying platform.

[navidshaikh]

client/pkg/kn/commands/plugin/plugin_verifier.go

Lines 138 to 146 in c4e8d5a

	var sys *syscall.Stat_t
	var ok bool
	if sys, ok = fileInfo.Sys().(*syscall.Stat_t); !ok {
	// We can check the files' owner/group
	return eaw.addWarning("cannot check owner/group of file %s", path)
	}
	isOwner := checkIfUserIsFileOwner(sys.Uid)
	isInGroup, err := checkIfUserInGroup(sys.Gid)

Apparently, this part is not portable, https://groups.google.com/forum/#!topic/golang-nuts/ywS7xQYJkHY