Set :proc_lib initial call on task

Sets the `:proc_lib` initial call to the `{mod, fun, arity}` of the task.

This allows `:proc_lib.translate_initial_call(task.pid)` to show the `mfa` of a task process when debugging. Tools such as `:observer` and the erlang shell make use of this function when getting information on a process.

This gives a useful initial call in the crash report sent to the error logger when a task raises.