check-incremental: explicitly use python3 (#41047)
check-incremental: explicitly use python3 (#41047)

Addresses rdar://88092462
benlangmuir/swift,atrick/swift,benlangmuir/swift,apple/swift,glessard/swift,roambotics/swift,ahoppen/swift,apple/swift,gregomni/swift,glessard/swift,ahoppen/swift,gregomni/swift,rudkx/swift,benlangmuir/swift,apple/swift,glessard/swift,rudkx/swift,atrick/swift,apple/swift,roambotics/swift,ahoppen/swift,gregomni/swift,glessard/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,atrick/swift,atrick/swift,JGiola/swift,roambotics/swift,gregomni/swift,roambotics/swift,rudkx/swift,apple/swift,rudkx/swift,atrick/swift,JGiola/swift,JGiola/swift,ahoppen/swift,benlangmuir/swift,JGiola/swift,glessard/swift,JGiola/swift,roambotics/swift,gregomni/swift,benlangmuir/swift,rudkx/swift,rudkx/swift,glessard/swift,gregomni/swift,atrick/swift,ahoppen/swift,apple/swift,ahoppen/swift