cmd/go: prefer 'go get -d' instead of 'go get' in script tests

'get -d' has somewhat narrower semantics and is generally faster.
We're deprecating the non-'-d' mode in CL 266360.

For #26472

Change-Id: Id4a324771f77b83e5f47043fd50b74e1c062390b
Reviewed-on: https://go-review.googlesource.com/c/go/+/267883


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