On modern Unix systems it is basically impossible for a multithreaded
program to open a binary for write, close it, and then fork+exec that
same binary. So don't write the binary if we're going to fork+exec it.

This fixes the ETXTBSY flakes.

Fixes #22220.
See also #22315.

Change-Id: I6be4802fa174726ef2a93d5b2f09f708da897cdb
Reviewed-on: https://go-review.googlesource.com/71570
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>