Arduino HF Interference Protection
IMPEDANCE - Use a very low input impedance so that it responds well to a "dead short" like a switch, but is "de-sensitized" to random EMI floating around in the air.  As FakeMoustache suggested perhaps using a pull-up resistor around 1K would make it nice and "stiff".  And even some significant capacitance on the order on 1nF wouldn't hurt because you are dealing with a very low-speed circuit here and it might even benefit from some "debounce" help.
FILTERING - You can use parallel capacitance and series inductance to block high frequencies (like RFI) while still allowing DC (like from your switch) to work normally.  For example look up a "pi-filter"  (nothing to do with the Raspberry Pi).
ISOLATION - In extreme conditions (unlikely in this case) sometmes techniques such as optical isolation are used to further decouple the control electronics from long distance signal connections through "hostile territories".


Ref: https://en.wikipedia.org/wiki/Capacitor-input_filter