cmd: refresh cmd/vendor to match 'go mod vendor'

cmd: refresh cmd/vendor to match 'go mod vendor'

This change preserves the maximum versions from cmd/vendor/vendor.json
where feasible, but bumps the versions of x/sys (for CL 162987) and
x/tools (for CL 162989 and CL 160837) so that 'go test all' passes in
module mode when run from a working directory in src/cmd.

A small change to cmd/vet (not vendored) was necessary to preserve its
flag behavior given a pristine copy of x/tools; see CL 162989 for more
detail.

This change was generated by running 'go mod vendor' at CL 164622.
(Welcoooome to the fuuuuuture!)

Updates #30228
Updates #30241

Change-Id: I889590318dc857d4a6e20c3023d09a27128d8255
Reviewed-on: https://go-review.googlesource.com/c/go/+/164618
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>