Properly detect Zsh shell

There is no need to define custom shell var, when Zsh can be detected by
checking SHELL env var.

help.go

@@ -102,7 +102,7 @@ func printCommandSuggestions(commands []*Command, writer io.Writer) {
 		if command.Hidden {
 			continue
 		}
-		if os.Getenv("_CLI_ZSH_AUTOCOMPLETE_HACK") == "1" {
+		if strings.Contains(os.Getenv("SHELL"), "zsh") {
 			for _, name := range command.Names() {
 				_, _ = fmt.Fprintf(writer, "%s:%s\n", name, command.Usage)
 			}