[chore] Inform user on how to get testrig (#1557)

This adds a dummy testrig subcommand when we're not building with DEBUG
set. Now any invocation of gotosocial testrig, plain or with any kind of
subcommands or arguments will result in that error message getting
returned.

cmd/gotosocial/main.go

@@ -20,6 +20,7 @@ package main
 
 import (
 	"log"
+	"os"
 	godebug "runtime/debug"
 	"strings"
 
@@ -66,6 +67,8 @@ func main() {
 	if debug.DEBUG {
 		// only add testrig if debug enabled.
 		rootCmd.AddCommand(testrigCommands())
+	} else if len(os.Args) > 1 && os.Args[1] == "testrig" {
+		log.Fatalln("gotosocial must be built and run with the DEBUG enviroment variable set to enable and access testrig")
 	}
 
 	// run