package/bash: use target finalize hook

Arnout reported an issue with the upcoming top-level parallel build.
The file /etc/shells can be modified by several packages (shell providers)

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>