cmd/doc: properly set GOPROXY to avoid deprecation checks
This change fixes a bug that was introduced in CL 675155. Instead of
doing the two step download and run with GOPROXY=off, do the run with
GOPROXY=<download cache>:$GOPROXY, so that we use the previously
downloaded version of pkgsite as the latest.
Fixes #73833
Change-Id: I8803426498ab026602805d6448a130eb11458c99
Reviewed-on: https://go-review.googlesource.com/c/go/+/675576 Reviewed-by: Jonathan Amsterdam <jba@google.com> Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>