So you want to have the PIR work in the dark only, and switch on a latching relay, that stays on until you press a button (momentary switch).
So the only thing the Arduino has to do, is detect dark/light and switch on the PIR sensor accordingly, right? Sounds a very overengineered solution to just adding the LDR to the PIR sensor directly. If you want better control for the darkness level at which it switches - which is based on the resistance of the LDR, low resistance is light, high resistance is dark - it may work to add a 100k pot.
Then all that's left is the latching relay, that's also a routine circuit. Easiest is if you have a double relay, where one set of poles switches the light, the other set latches the relay itself.
Like this:
Note: the NPN transistor is probably built into your relay module already, together with the base resistor.