cmd/go: make cfg.BuildContext.ToolTags same order with build.Default.ToolTags

cmd/go: make cfg.BuildContext.ToolTags same order with build.Default.ToolTags

So it's consistent when running "go list -f '{{context.ToolTags}}'" and
printing the content of "build.Default.ToolTags".

Updates #45454

Change-Id: I7a3cbf3cdf9a6ce2b8c89e9bcf5fc5e9086d48e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/422615
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>