cerbero: The toolchain_prefix is not always defined
cerbero: The toolchain_prefix is not always defined

Fixes all CI:

TypeError: stat: can't specify None for path argument

GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,fluendo/cerbero,atsushieno/cerbero,GStreamer/cerbero,GStreamer/cerbero,fluendo/cerbero,fluendo/cerbero,nirbheek/cerbero,atsushieno/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,fluendo/cerbero,fluendo/cerbero,atsushieno/cerbero,atsushieno/cerbero,atsushieno/cerbero