[dev.typeparams] cmd/dist: disable -G=3 on the std go tests for now

Disable -G=3 tests on the std go tests, in order to see if -G=3 is
causing the flakiness for the dev.typeparams builder, as opposed to
other changes in typeparams branch. It's possible that -G=3 is using
more CPU/RAM that causes flakiness, as opposed to more specific bugs.

Change-Id: I610bce2aabd26b2b1fddc5e63f85ffe4e958e0d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/292850


TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>