/tmp/.crank/wrk shared by all crank users

When using the wrk load generator, crank mkdir /tmp/.crank and then writes an executable by get from some URL.

Unfortunately, the permissions of the last user for /tmp/.crank may preclude the current user from copying in a new version of wrk.

/tmp/.crank directory name should also encode the user's name
@sebastienros
@EgorBo