build: try harder to pass along CMAKE_Swift_COMPILER
build: try harder to pass along CMAKE_Swift_COMPILER

This is needed for the CMake 3.15 based build.

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