release: Fix typo in powerpc mkisoimages.sh

"[ n foo ]" is an error and should have been "[ -n foo ]".

Fortunately the only ill effect was to result in garbage in /tmp/ when
creating powerpc ISO images.

Fixes: 6c3e01bf8578 ("release: Randomize powerpc boot block file name")
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva