Separate cached kubernetes binaries by OS on host machine

This way, all kubernetes binaries stored in ~/.minikube/cache/linux will be copied over to the VM and used for `minikube kubectl` on linux machines. kubectl will be stored in `~/.minikube/cache/{windows or darwin}` on windows/darwin so that `minikube kubectl` still works.

pkg/minikube/machine/cache_binaries.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"os"
 	"path"
+	"path/filepath"
 	"runtime"
 
 	"github.com/golang/glog"
@@ -63,7 +64,7 @@ func KubernetesReleaseURLSHA1(binaryName, version, osName, archName string) stri
 
 // CacheBinary will cache a binary on the host
 func CacheBinary(binary, version, osName, archName string) (string, error) {
-	targetDir := localpath.MakeMiniPath("cache", version)
+	targetDir := localpath.MakeMiniPath(filepath.Join("cache", osName), version)
 	targetFilepath := path.Join(targetDir, binary)
 
 	url := KubernetesReleaseURL(binary, version, osName, archName)