build: repair cross-compilation for Windows after #33908
build: repair cross-compilation for Windows after #33908

`CMAKE_SYSTEM_NAME` specifies the host, not the build.  Use the correct
executable extension for the host.

parkera/swift,jmgc/swift,jmgc/swift,nathawes/swift,jmgc/swift,atrick/swift,hooman/swift,jmgc/swift,ahoppen/swift,roambotics/swift,ahoppen/swift,xwu/swift,xwu/swift,gregomni/swift,hooman/swift,glessard/swift,ahoppen/swift,atrick/swift,tkremenek/swift,glessard/swift,rudkx/swift,atrick/swift,hooman/swift,jmgc/swift,xwu/swift,atrick/swift,parkera/swift,JGiola/swift,JGiola/swift,tkremenek/swift,nathawes/swift,gregomni/swift,roambotics/swift,xwu/swift,parkera/swift,benlangmuir/swift,gregomni/swift,nathawes/swift,gregomni/swift,atrick/swift,benlangmuir/swift,JGiola/swift,atrick/swift,parkera/swift,glessard/swift,roambotics/swift,JGiola/swift,hooman/swift,JGiola/swift,nathawes/swift,hooman/swift,nathawes/swift,rudkx/swift,hooman/swift,jmgc/swift,apple/swift,gregomni/swift,tkremenek/swift,roambotics/swift,benlangmuir/swift,tkremenek/swift,benlangmuir/swift,tkremenek/swift,apple/swift,xwu/swift,rudkx/swift,parkera/swift,apple/swift,nathawes/swift,roambotics/swift,apple/swift,jmgc/swift,gregomni/swift,rudkx/swift,benlangmuir/swift,parkera/swift,glessard/swift,parkera/swift,apple/swift,rudkx/swift,apple/swift,glessard/swift,parkera/swift,glessard/swift,ahoppen/swift,rudkx/swift,ahoppen/swift,nathawes/swift,tkremenek/swift,xwu/swift,ahoppen/swift,roambotics/swift,JGiola/swift,hooman/swift,xwu/swift,benlangmuir/swift,tkremenek/swift