Make use of %USERPROFILE% in cli help message

An earlier commit was causing docker windows CLI build to not to pick up
the shorthand form for home directory (`%USERPROFILE%`) shown in when
`docker --help` is executed.

Fixing that bug and making the if statement concise and clear.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

pkg/mflag/flag.go

@@ -506,15 +506,11 @@ func Set(name, value string) error {
 // otherwise, the default values of all defined flags in the set.
 func (f *FlagSet) PrintDefaults() {
 	writer := tabwriter.NewWriter(f.Out(), 20, 1, 3, ' ', 0)
-	var home string
-	if runtime.GOOS != "windows" {
-		// Only do this on non-windows systems
-		home = homedir.Get()
+	home := homedir.Get()
 
-		// Don't substitute when HOME is /
-		if home == "/" {
-			home = ""
-		}
+	// Don't substitute when HOME is /
+	if runtime.GOOS != "windows" && home == "/" {
+		home = ""
 	}
 	f.VisitAll(func(flag *Flag) {
 		format := "  -%s=%s"