build: be a bit more resilient
build: be a bit more resilient

In the case that a system update temporarily breaks the command, the output may
be empty, and this can cause CMake to fail in a weird manner.  Quote the
variable to be more resilient to the transient failure.

danielmartin/swift,airspeedswift/swift,xwu/swift,shajrawi/swift,allevato/swift,glessard/swift,gribozavr/swift,shahmishal/swift,nathawes/swift,danielmartin/swift,practicalswift/swift,airspeedswift/swift,airspeedswift/swift,allevato/swift,jckarter/swift,austinzheng/swift,JGiola/swift,alblue/swift,karwa/swift,JGiola/swift,amraboelela/swift,shahmishal/swift,austinzheng/swift,CodaFi/swift,jckarter/swift,karwa/swift,sschiau/swift,JGiola/swift,xedin/swift,sschiau/swift,lorentey/swift,xwu/swift,CodaFi/swift,roambotics/swift,alblue/swift,lorentey/swift,ahoppen/swift,ahoppen/swift,aschwaighofer/swift,danielmartin/swift,stephentyrone/swift,rudkx/swift,brentdax/swift,practicalswift/swift,stephentyrone/swift,gregomni/swift,JGiola/swift,jmgc/swift,alblue/swift,shahmishal/swift,karwa/swift,hooman/swift,aschwaighofer/swift,apple/swift,shajrawi/swift,hooman/swift,nathawes/swift,parkera/swift,glessard/swift,glessard/swift,roambotics/swift,stephentyrone/swift,devincoughlin/swift,atrick/swift,gregomni/swift,shajrawi/swift,ahoppen/swift,CodaFi/swift,CodaFi/swift,tkremenek/swift,hooman/swift,atrick/swift,aschwaighofer/swift,austinzheng/swift,atrick/swift,amraboelela/swift,austinzheng/swift,shajrawi/swift,lorentey/swift,JGiola/swift,ahoppen/swift,devincoughlin/swift,jmgc/swift,sschiau/swift,CodaFi/swift,xedin/swift,devincoughlin/swift,shajrawi/swift,apple/swift,brentdax/swift,allevato/swift,amraboelela/swift,gribozavr/swift,lorentey/swift,harlanhaskins/swift,aschwaighofer/swift,gribozavr/swift,practicalswift/swift,rudkx/swift,devincoughlin/swift,roambotics/swift,ahoppen/swift,jckarter/swift,xwu/swift,xwu/swift,danielmartin/swift,hooman/swift,devincoughlin/swift,hooman/swift,shahmishal/swift,harlanhaskins/swift,xedin/swift,aschwaighofer/swift,allevato/swift,amraboelela/swift,glessard/swift,nathawes/swift,brentdax/swift,karwa/swift,tkremenek/swift,stephentyrone/swift,devincoughlin/swift,atrick/swift,tkremenek/swift,austinzheng/swift,brentdax/swift,gribozavr/swift,amraboelela/swift,nathawes/swift,shajrawi/swift,allevato/swift,austinzheng/swift,practicalswift/swift,shahmishal/swift,alblue/swift,lorentey/swift,nathawes/swift,jckarter/swift,xwu/swift,apple/swift,jckarter/swift,benlangmuir/swift,parkera/swift,practicalswift/swift,xedin/swift,gribozavr/swift,aschwaighofer/swift,gregomni/swift,glessard/swift,harlanhaskins/swift,jmgc/swift,danielmartin/swift,xedin/swift,atrick/swift,harlanhaskins/swift,parkera/swift,parkera/swift,parkera/swift,tkremenek/swift,brentdax/swift,harlanhaskins/swift,shahmishal/swift,sschiau/swift,jckarter/swift,airspeedswift/swift,brentdax/swift,glessard/swift,roambotics/swift,tkremenek/swift,gregomni/swift,jckarter/swift,parkera/swift,atrick/swift,xwu/swift,JGiola/swift,xedin/swift,amraboelela/swift,practicalswift/swift,apple/swift,amraboelela/swift,ahoppen/swift,shahmishal/swift,danielmartin/swift,shahmishal/swift,tkremenek/swift,danielmartin/swift,jmgc/swift,practicalswift/swift,jmgc/swift,alblue/swift,rudkx/swift,lorentey/swift,apple/swift,gregomni/swift,tkremenek/swift,rudkx/swift,aschwaighofer/swift,sschiau/swift,xwu/swift,sschiau/swift,xedin/swift,devincoughlin/swift,stephentyrone/swift,stephentyrone/swift,devincoughlin/swift,benlangmuir/swift,benlangmuir/swift,gregomni/swift,benlangmuir/swift,airspeedswift/swift,apple/swift,benlangmuir/swift,austinzheng/swift,CodaFi/swift,CodaFi/swift,benlangmuir/swift,practicalswift/swift,roambotics/swift,lorentey/swift,jmgc/swift,gribozavr/swift,sschiau/swift,allevato/swift,jmgc/swift,gribozavr/swift,shajrawi/swift,hooman/swift,alblue/swift,shajrawi/swift,harlanhaskins/swift,karwa/swift,airspeedswift/swift,airspeedswift/swift,hooman/swift,lorentey/swift,harlanhaskins/swift,brentdax/swift,parkera/swift,alblue/swift,rudkx/swift,nathawes/swift,nathawes/swift,karwa/swift,allevato/swift,sschiau/swift,karwa/swift,karwa/swift,gribozavr/swift,rudkx/swift,roambotics/swift,parkera/swift,stephentyrone/swift,xedin/swift