cmd/go/testdata: update addmod.go to work with the mainline go command

Updates #32715

Change-Id: I4ea41d7bd1a2daeba8cebb283aaa096b6c3d4533
Reviewed-on: https://go-review.googlesource.com/c/go/+/183401
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/cmd/go/testdata/addmod.go

@@ -29,7 +29,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	"../internal/txtar"
+	"cmd/go/internal/txtar"
 )
 
 func usage() {
@@ -44,7 +44,7 @@ func fatalf(format string, args ...interface{}) {
 	log.Fatalf(format, args...)
 }
 
-const goCmd = "vgo"
+const goCmd = "go"
 
 func main() {
 	flag.Usage = usage