Comment 4 for bug 254266

Your postinst seems to be fine.
Can you check if that file and /var/lib/dpkg/info/ have the executable bit set? Can you check if /var is mounted with the exec flag set?
Can you try to manually execute that script?
If worst come to worse, you can try to force the installation with:

sudo dpkg --force-all -i /var/cache/apt/archives/libgnome-cni_2.12.7-4_powerpc.deb

Its a dangerous command to give, so try other solutions first.
This may not solve others problem you have which could be related to a corrupt file system but at least should solve this one.
Anyhow, I'm transforming this into a question since there doesn't seem to be any bug.