[windows] Update ICU version to 64.2.
[windows] Update ICU version to 64.2.

Improve a little the build script to not have to change the version in
two places of the URL.

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