Set _builddir and _sourcedir to the temporary directory for spec file eval
We generally don't want to reach into ~/rpmbuild/SOURCES or
~/rpmbuild/BUILD when evaluating the specfile in a temporary directory.
Previous commit removed %include, but let's make sure that other
references to sources or patches don't look in an unexpected place.
Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>