arch-nspawn: allow oddly named directories

This fixes various errors one might encounter when trying to use a
build root or cachedir with whitespace in it.

Note that the cachedir fix is not a complete one, as pacman's output is
unreliable (and not meant for parsing here).

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>