cmd/go: eliminate empty '()' when passing -mod=readonly explicitly to…

[sthagen]

… 'go list'

Discovered while investigating golang#37197.

Updates golang#33326
Updates golang#34822

Change-Id: I38b136a4ee762a580a554125066b9778491295f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/219237
Run-TryBot: Bryan C. Mills bcmills@google.com
Reviewed-by: Michael Matloob matloob@golang.org
TryBot-Result: Gobot Gobot gobot@golang.org

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