cmd/dist: revert mod=vendor change accidentally included in CL 164623

I thought I had removed all of the 'mod=vendor' hacks I had inserted
previously, but apparently missed this one in a bad merge or rebase.

Updates #30228

Change-Id: Ia5aea754bf986458373fefd08fa9dd3941e31e43
Reviewed-on: https://go-review.googlesource.com/c/go/+/167077


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>