change /cache/recovery group to cache

To unbundle system update, we need users other than system to be able
to write the /cache/recovery directory.
diff --git a/rootdir/init.rc b/rootdir/init.rc
index f62d1d8..cd3fd9e 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -97,7 +97,7 @@
     chmod 0770 /cache
 
     # This may have been created by the recovery system with odd permissions
-    chown system system /cache/recovery
+    chown system cache /cache/recovery
     chmod 0770 /cache/recovery
 
     #change permissions on vmallocinfo so we can grab it from bugreports