cmd/go/internal/mvs: reverse the order of BuildListError.stack

When we print the stack from a BuildListError, we print the main
module first and the error last. That was the opposite of the order in
which in was stored in memory, leading to (arguably) more complex code
and (definitely) my own inability to reason about the contents of the
slice.

For now, it's still more convenient to construct the stack reversed,
so we do that and then reverse it before packing it into the error.

For #36460

Change-Id: I6312fb67b2ad9bf9b64071fe829854833208bad7
Reviewed-on: https://go-review.googlesource.com/c/go/+/244759


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>