cmd/vendor: go get -u golang.org/x/tools && go mod vendor

To pick up the structtag vet fix for 1.13.

Fixes #30846.

Change-Id: I5e011a7db1ffb9435793d533097d768f209c18e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/179999


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>