cmd/vendor: pull in the x/mod/modfile ignore directive

go get golang.org/x/mod@9d33331
go mod tidy
go mod vendor

For #42965

Change-Id: Ib741c96be53214c35058a564116688e7122a205a
Reviewed-on: https://go-review.googlesource.com/c/go/+/670975


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>