cmd/go: don't use a testlog if there is an exec command

cmd/go: don't use a testlog if there is an exec command

An exec command is normally used on platforms were the test is run in
some unusual way, making it less likely that the testlog will be useful.

Updates #22593

Change-Id: I0768f6da89cb559d8d675fdf6d685db9ecedab9e
Reviewed-on: https://go-review.googlesource.com/83578
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>