IDEA-175506 Build scripts: download JREs only for platforms for which product is being built

pass targetOs property to gradle task

build/dependencies/setupJbre.gradle

@@ -1,6 +1,7 @@
 task setupJbre {}
 
 def jdkRepo = inJetBrainsNetwork ? 'http://repo.labs.intellij.net/intellij-jdk' : 'https://dl.bintray.com/jetbrains/intellij-jdk'
+def targetOs = "${System.getProperty("intellij.build.target.os", "all")}"
 
 jrePlatformsToDownload(targetOs).each { platform ->
   archToDownload(platform).each { arch ->

build/dependencies/setupJdk.gradle

@@ -22,7 +22,6 @@ def checkoutDir = intellijProjectDir()
 def jdkDir = "$checkoutDir/build/jdk"
 
 project.ext.jdk18OutputDir = "${System.getProperty("intellij.jdk18.output.dir", "") ?: "$jdkDir/1.8"}"
-project.ext.targetOs = "${System.getProperty("intellij.build.target.os", "all")}"
 
 task downloadJdk18 {
   def arch = jdk18Arch()

platform/build-scripts/groovy/org/jetbrains/intellij/build/impl/BuildTasksImpl.groovy

@@ -296,7 +296,7 @@ idea.fatal.error.notification=disabled
       if (buildContext.productProperties.scrambleMainJar) {
         scramble()
       }
-      buildContext.gradle.run('Setting up JetBrains JREs', 'setupJbre')
+      buildContext.gradle.run('Setting up JetBrains JREs', 'setupJbre', "-Dintellij.build.target.os=$buildContext.options.targetOS")
       layoutShared()
 
       def propertiesFile = patchIdeaPropertiesFile()