cmd/go: reorder parameters so that context is first

cmd/go: reorder parameters so that context is first

This change simply reorders parameters so that the context.Context is
the first, as per standard practice.

This commit is part of the overall effort to eliminate global
modloader state.

Change-Id: I22d366fb2d2457f44d668409da3fe76fb87180cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/714780
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>