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
diff --git a/cc/config/x86_darwin_host.go b/cc/config/x86_darwin_host.go
index 1035df3..b0344af 100644
--- a/cc/config/x86_darwin_host.go
+++ b/cc/config/x86_darwin_host.go
@@ -136,7 +136,7 @@
 
 func getMacTools(ctx android.PackageVarContext) *macPlatformTools {
 	macTools.once.Do(func() {
-		xcrunTool := ctx.Config().NonHermeticHostSystemTool("xcrun")
+		xcrunTool := "/usr/bin/xcrun"
 
 		xcrun := func(args ...string) string {
 			if macTools.err != nil {