fix 10020_3dr_quad script: load proper mixer file (#4880)
fix 10020_3dr_quad script: load proper mixer file (#4880)

fixes regression from 85245471c05a7453ffa50e2e74dbf8f48c01982b
darknight-007/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,darknight-007/Firmware,jlecoeur/Firmware,PX4/Firmware,PX4/Firmware,Aerotenna/Firmware,dagar/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,darknight-007/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,dagar/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,acfloria/Firmware,darknight-007/Firmware,dagar/Firmware,krbeverx/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,krbeverx/Firmware,jlecoeur/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware