lib/common.sh: Revise setup_workdir()

* Honor TMPDIR variable (just like we did in commit a0c6bf45

).
* Quote path properly.
* Avoid unnecessary use of basename(1).

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>