cmd/dist: revert mod=vendor change accidentally included in CL 164623
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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

CAFxX/go,CAFxX/go,josharian/go,josharian/go,nawawi/go,josharian/go,nawawi/go,josharian/go,akutz/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,golang/go,akutz/go,akutz/go,golang/go,cloudflare/go,cloudflare/go,golang/go,josharian/go,CAFxX/go,akutz/go,nawawi/go,josharian/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,golang/go,josharian/go,josharian/go