Add exports to change default output directories of makechrootpkg

Hi graysky, clean-chroot-manager is a cool script. Thanks for sharing :)

A small gripe: makechrootpkg dumps everything in the PKGBUILD folder. I prefer to keep that folder clean, so I keep all build outputs in their own folders (using makepkg.conf). This patch adds that ability to  clean-chroot-manager too.

See if this is cool enough to add.

Cheers!