How to set XDG_RUNTIM_DIR environmental variable?

I'm working on a Raspberry Pi 4 with Raspbian OS. When I try to execute a python script, I obtain the error: XDG_RUNTIME_DIR not set in the environment. How can I fix this issue? I'm working with an ArduCAM multicamera module (https://github.com/ArduCAM/RaspberryPi/blob/master/Multi_Camera_Adapter/Multi_Adapter_Board_4Channel/Multi_Camera_Adapter_V2.2_python/) and this is the script (called by init_camera.sh):

import RPi.GPIO as gp
import os

gp.setwarnings(False)
gp.setmode(gp.BOARD)

gp.setup(7, gp.OUT)
gp.setup(11, gp.OUT)
gp.setup(12, gp.OUT)


def main():
    print('Start testing the camera A')
    i2c = "i2cset -y 1 0x70 0x00 0x04"
    os.system(i2c)
    gp.output(7, False)
    gp.output(11, False)
    gp.output(12, True)
    capture(1)
    print('Start testing the camera B') 
    i2c = "i2cset -y 1 0x70 0x00 0x05"
    os.system(i2c)
    gp.output(7, True)
    gp.output(11, False)
    gp.output(12, True)
    capture(2)
    print('Start testing the camera C')
    i2c = "i2cset -y 1 0x70 0x00 0x06"
    os.system(i2c)
    gp.output(7, False)
    gp.output(11, True)
    gp.output(12, False)
    capture(3)
    print('Start testing the camera D')
    i2c = "i2cset -y 1 0x70 0x00 0x07"
    os.system(i2c)
    gp.output(7, True)
    gp.output(11, True)
    gp.output(12, False)
    capture(4)
    
def capture(cam):
    cmd = "libcamera-still -o capture_%d.jpg" % cam
    os.system(cmd)

if __name__ == "__main__":
    main()

    gp.output(7, False)
    gp.output(11, False)
    gp.output(12, True)