cerbero: Cache the return value of config.get_env
cerbero: Cache the return value of config.get_env

The values will always be the same, so cache them. This speeds up
recipe parsing from 5.6s to 5.0s.

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