Is it Safe to Have an Attic Light & Receptacle on 60A HVAC Circuit
This is an unmitigated disaster

Firstly, 60A auxiliary heater circuits are normal enough, but they don't have neutral.   The wires are L1-L2-ground or black-red-bare.   There is no way to get 120V out of that circuit, except by bootlegging neutral. 

Bootlegging neutrl means misusing ground as neutral, intentionally connecting a "hot" wire (via a light bulb or tool) to the ground.   If the ground-neutral bond in your panel has any sort of problem, it will electrify every ground in your house.  Bad idea. 

Nothing prevents someone from plugging in a power strip and four 1500W heaters.  That will overheat the wiring and start a fire. 

This is terrible work! 

Must be more going on

Thing with heat pumps is there are usually other circuits involved.  I am surprised that the air handler would share the 60A aux heat circuit, I would expect there to be a 15-20A circuit there for the air handler.  I would think it might also be 120V.   If so, it may be possible to borrow it for the attic light.   The attic light must be on a 15 or 20A 120V circuit.  It cannot use part of a 120/240 circuit. 

The total of all hard-wired loads on a circuit must be less than 80% of total circuit capacity.   

You cannot have any receptacles on a circuit unless the hard-wired loads are less than 50% of total circuit capacity.  So you may need to change lamp fixtures. 

Or, go 24V lighting

In this day and age, LEDs are so efficient that 5W of lighting is a fair bit, and two would light up an attic decently enough.   Most furnaces have a 24 volt AC, 40VA (similar to watts) transformer with surplus capacity.   You could grab 24VAC off the transformer and take it to LED lights.  You would need to find LED fixtures that have no replaceable bulb, and work a range of voltages including 24V AC.  You can use common thermostat wire to hook them up.  Aim for an even number of lights, and hook each one reverse polarity of the next.  Sometimes LEDs will tolerate AC but only light on half the AC cycle, by reversing polarity to every other one, you assure the full cycle is used.