build: explicitly pass the path to python2,python3
build: explicitly pass the path to python2,python3

Explicitly pass the path to the python interpreter on macOS by using
`xcrun` to find the tools.

CodaFi/swift,JGiola/swift,allevato/swift,atrick/swift,parkera/swift,roambotics/swift,CodaFi/swift,allevato/swift,benlangmuir/swift,rudkx/swift,allevato/swift,xwu/swift,CodaFi/swift,ahoppen/swift,hooman/swift,harlanhaskins/swift,gregomni/swift,benlangmuir/swift,xwu/swift,aschwaighofer/swift,jckarter/swift,nathawes/swift,hooman/swift,tkremenek/swift,ahoppen/swift,nathawes/swift,glessard/swift,parkera/swift,ahoppen/swift,jckarter/swift,airspeedswift/swift,rudkx/swift,aschwaighofer/swift,apple/swift,glessard/swift,jmgc/swift,benlangmuir/swift,glessard/swift,aschwaighofer/swift,xwu/swift,allevato/swift,stephentyrone/swift,tkremenek/swift,aschwaighofer/swift,benlangmuir/swift,parkera/swift,jmgc/swift,parkera/swift,harlanhaskins/swift,xwu/swift,rudkx/swift,apple/swift,stephentyrone/swift,ahoppen/swift,atrick/swift,jckarter/swift,airspeedswift/swift,JGiola/swift,apple/swift,roambotics/swift,rudkx/swift,JGiola/swift,allevato/swift,tkremenek/swift,roambotics/swift,xwu/swift,gregomni/swift,CodaFi/swift,airspeedswift/swift,jmgc/swift,harlanhaskins/swift,harlanhaskins/swift,hooman/swift,atrick/swift,tkremenek/swift,nathawes/swift,aschwaighofer/swift,apple/swift,gregomni/swift,parkera/swift,tkremenek/swift,atrick/swift,rudkx/swift,allevato/swift,stephentyrone/swift,aschwaighofer/swift,nathawes/swift,xwu/swift,atrick/swift,benlangmuir/swift,nathawes/swift,CodaFi/swift,stephentyrone/swift,gregomni/swift,glessard/swift,roambotics/swift,jmgc/swift,CodaFi/swift,roambotics/swift,airspeedswift/swift,JGiola/swift,hooman/swift,tkremenek/swift,airspeedswift/swift,ahoppen/swift,nathawes/swift,atrick/swift,aschwaighofer/swift,airspeedswift/swift,apple/swift,stephentyrone/swift,allevato/swift,gregomni/swift,jckarter/swift,parkera/swift,xwu/swift,airspeedswift/swift,benlangmuir/swift,hooman/swift,harlanhaskins/swift,tkremenek/swift,jmgc/swift,stephentyrone/swift,glessard/swift,jmgc/swift,harlanhaskins/swift,hooman/swift,rudkx/swift,jckarter/swift,harlanhaskins/swift,glessard/swift,stephentyrone/swift,JGiola/swift,gregomni/swift,CodaFi/swift,ahoppen/swift,roambotics/swift,apple/swift,nathawes/swift,hooman/swift,jckarter/swift,jckarter/swift,jmgc/swift,parkera/swift,parkera/swift,JGiola/swift