makechrootpkg: use var instead of file in $copydir

Signed-off-by: Sébastien Luttringer <seblu@seblu.net>
makechrootpkg: Remove btrfs copy after successful build

Avoid to manually cleanup of subvolumes when a build was successful.
Really useful with random copy directory.

Signed-off-by: Sébastien Luttringer <seblu@seblu.net>