Drop keepnr from 16 to 12

Space is running out…

jobsets.nix

@@ -9,7 +9,7 @@ let
     schedulingshares = 100;
     enableemail = true;
     emailoverride = "";
-    keepnr = 16;
+    keepnr = 12;
     type = 0;
     inputs = {
       hydra-config = {