Make the shebang line portable (Closes #1)

nss-systemcerts-import erroneously pointed to the Bourne Shell,
and nss-systemcerts-hook pointed to /bin/bash, which may not be
the correct location on all systems.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Raphaël Barrois <raphael.barrois@polyconseil.fr>