cmd/dist: only build exec wrappers when cross compiling

Updates #31722

Change-Id: Ib44b46e628e364fff6eacda2b26541db2f0a4261
Reviewed-on: https://go-review.googlesource.com/c/go/+/174701
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/build.go

@@ -1425,6 +1425,9 @@ func cmdbootstrap() {
 }
 
 func wrapperPathFor(goos, goarch string) string {
+	if goos == gohostos && goarch == gohostarch {
+		return ""
+	}
 	switch {
 	case goos == "android":
 		return pathf("%s/misc/android/go_android_exec.go", goroot)