cmd: make go and moddeps tests insensitive to presence of src/go.work

cmd: make go and moddeps tests insensitive to presence of src/go.work

If there is a go.work file in src, "go test cmd/internal/moddeps"
and "go test cmd/go" fail.  Setting GOWORK=off in various command
invocations avoids this problem.

Change-Id: I89fd021ff94998ceda6a3bbc4c3743f08558f98b
Reviewed-on: https://go-review.googlesource.com/c/go/+/396777
Trust: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>