Revert "Allow using /bin and /usr/bin as impure prefixes on non-darwin by default"

This reverts commit 79ca5033. Ouch,
never noticed this. We definitely don't want to allow builds to have
arbitrary access to /bin and /usr/bin, because then they can (for
instance) bring in a bunch of setuid programs. Also, we shouldn't be
encouraging the use of impurities in the default configuration.