[pkg] Use the new deprecated_system_package alias

PKG-46

Change-Id: If40ad522ddc1956e60671f80d4326482991e5851
diff --git a/BUILD.gn b/BUILD.gn
index b2f2dc1..8e0f82f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -174,7 +174,7 @@
 
 package("re2_tests") {
   testonly = true
-  system_image = true
+  deprecated_system_image = true
 
   deps = [
     ":re2_slow_tests",