dist: generate stub go.mod in workdir

...and run commands from there.

This removes the requirement that bootstrap must not run inside a module
by ensuring that enclosing modules do not interfere with bootstrap.

Fixes #44209.

Change-Id: I700a81829226770b8160c8ff04127b855b6e26bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/296610


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