On Plan 9, GNU diff is called ape/diff.

Fixes #18999.

Change-Id: I7cf6c23c97bcc47172bbf838fd9dd72aefa4c18b
Reviewed-on: https://go-review.googlesource.com/36650Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>