cmd/go: set go.work path using GOWORK, and remove -workfile flag

This change removes the -workfile flag and allows the go.work file path
to be set using GOWORK (which was previously read-only). This removes
the potential discrepancy and confusion between the flag and environment
variable.

GOWORK will still return the actual path of the go.work file found if it
is set to '' or 'auto'. GOWORK will return 'off' if it is set to 'off'.

For #45713
Fixes #51171

Change-Id: I72eed65d47c63c81433f2b54158d514daeaa1ab3
Reviewed-on: https://go-review.googlesource.com/c/go/+/385995


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>