cmd/go: add -shift to go vet's flag whitelist

CL 40112 intended to allow full flag processing in go vet, but missed
vet's -shift flag; this corrects the omission.

Fixes #22442

Change-Id: I47525018306bd8b9aa452fb378d0d45319f8cf11
Reviewed-on: https://go-review.googlesource.com/73553
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/vet/vetflag.go

@@ -44,6 +44,7 @@ var vetFlagDefn = []*cmdflag.Defn{
 	{Name: "rangeloops", BoolVar: new(bool)},
 	{Name: "shadow", BoolVar: new(bool)},
 	{Name: "shadowstrict", BoolVar: new(bool)},
+	{Name: "shift", BoolVar: new(bool)},
 	{Name: "source", BoolVar: new(bool)},
 	{Name: "structtags", BoolVar: new(bool)},
 	{Name: "tests", BoolVar: new(bool)},