cmd/go: disable workspace mode in GOPATH mode

cmd/go: disable workspace mode in GOPATH mode

Workspace mode is specifically for working with modules;
it doesn't make sense in GOPATH mode.

This also fixes a panic in (*modload.MainModuleSet).GoVersion
when go.work is present in GOPATH mode.

For #58767.

Change-Id: Ic6924352afb486fecc18e009e6b517f078e81094
Reviewed-on: https://go-review.googlesource.com/c/go/+/471600
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>