B0_driver.EnvSourceDriver environment variables.
b0_dir is the variable used to specify the b0 directory.
b0_file is the variable used to specify the b0 file to use.
cache_dir is the variable used to specify the b0 cache directory.
code is the variable used to specify the code to which the driver compiles to.
hash_fun is the variable used to specify the cache hash function.
jobs is the variable used to specify the maximal number of spawns.