Relay switching a light causes a spike, making a raspberry pi halt
You should troubleshoot this in a couple of steps. 

First leave the circuit as-is but program the nano to turn the lamps on and off autonomously. The Pi should still be affected. 

Now disconnect the wiring between the nano and the Pi (using a different power supply if necessary). See if the Pi is still affected. My guess is that it will not be. 

If it is not affected then the problem is EMI caused by the inductive load of the lamps, particularly when the contacts open. You can try substituting an SSR or adding a snubber across the contacts, though the SSR is a better bet. The "Hockey puck" style is popular and probably does not require a heatsink for such a light load.