The vetx output file is a build output, and as such should be
deterministic. This CL changes it to not depend on map iteration order.

This avoids a pointless GODEBUG=gocacheverify=1 failure.

Updates #25666

Change-Id: Ic132bad134cb10938275f883c2c68432cb7c4409
Reviewed-on: https://go-review.googlesource.com/121941
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>