devtools: make sure all files are owned by nobody before building

In makechrootpkg, build dirs are chowned to nobody before we move files
to "$uniondir/build". This could lead to failure If the files are
supposed to be dynamically upgraded when we run makepkg.

A common use-case is when we upgrade $pkgver in git,svn PKGBUILDS.

Signed-off-by: Nezmer <Nezmer@allurelinux.org>
Signed-off-by: Aaron Griffin <aaronmgriffin@gmail.com>