run-in-tempdir: Improve speed

By creating symlinks instead of actually creating files we still get the
benefits of using a temporary directory, but the preparation is about
three times as fast. Test used:

$ time bash -c 'for ((i=0; i < 100; ++i)); do
    autotools/run-in-tempdir true; done'

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>