cmd: go get golang.org/x/tools@d88ec18 && go mod vendor

This brings in CL 261725, which applies the vet buildtag checker
to ignored files.

For #41184.

Change-Id: I5f241db5a71fa84d0af0b9e9f8a75ff5bb3dd116
Reviewed-on: https://go-review.googlesource.com/c/go/+/261958


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>