executor: mount bin and lib dirs to fuzzing sandbox

[benkittner]

Added bin and lib dirs to fuzzing sandbox to support system system calls that require provisioning utilities (fs)

[dvyukov]

It may be better to provide executor/common_ext.h extension point to alter the sandbox for forks. The list of dirs we need to mount can go on and on, and that unused clutter appears in reproducers sent to upstream kernel developers.