l3build: Add a short Winodws batch file demo

A wrapper of this form allows use of build.lua in the same way as is doable
on Unix by appling chmod.  Obvious question: do we want to add these abilities
to our own setup?


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5127 de43f980-851b-0410-b2f7-c40aca1f87e0