Hard-wire the path to xcrun as /usr/bin/xcrun.

This lets us not leak $PATH to soong_build. It was only needed for
Darwin tests anyway.

Test: Will ask jingwen@ who has a Mac.
Change-Id: I4647e41275b323fe6283580f8f92718c6229f23e

ui/build/soong.go

@@ -177,14 +177,6 @@ func runSoong(ctx Context, config Config) {
 		ninjaEnv.Set("TOP", os.Getenv("TOP"))
 		ninjaEnv.Set("SOONG_OUTDIR", config.SoongOutDir())
 
-		// Needed for NonHermeticHostSystemTool() and that, only in tests. We should
-		// probably find a better way of running tests other than making $PATH
-		// available also to production builds. Note that this is not get same as
-		// os.Getenv("PATH"): config.Environment() contains the $PATH that redirects
-		// every binary through the path interposer.
-		configPath, _ := config.Environment().Get("PATH")
-		ninjaEnv.Set("PATH", configPath)
-
 		// For debugging
 		if os.Getenv("SOONG_DELVE") != "" {
 			ninjaEnv.Set("SOONG_DELVE", os.Getenv("SOONG_DELVE"))