[pkg] Use the new deprecated_system_package alias

PKG-46

Change-Id: I0692cdea2fbd50e1911452d0ff9c738a18af946d
diff --git a/BUILD.gn b/BUILD.gn
index ee61655..b4e485d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -59,7 +59,7 @@
 
 package("libc-tests") {
   testonly = true
-  system_image = true
+  deprecated_system_image = true
 
   deps = [
     ":libc-large-tests",