multimedia/librav1e: unbreak PREFIX != LOCALBASE

$ poudriere testport -Pj 113i386 multimedia/librav1e
[...]
strip: open /wrkdirs/usr/ports/multimedia/librav1e/work/stage/prefix/librav1e-0.3.1_3/lib/*.so failed: No such file or directory

PR: 245536
Reported by: mi