Fix (make-)issuer.nix file name inconsistency

Discern make-issuer.nix from issuer.nix better.

Not necessary at all, but nice.

morph/grid/production/grid.nix

@@ -20,7 +20,7 @@ import ../../lib/make-grid.nix {
     # doesn't specify one.
     #
     # The names must be unique!
-    "payments.privatestorage.io" = import ../../lib/issuer.nix ({
+    "payments.privatestorage.io" = import ../../lib/make-issuer.nix ({
       publicIPv4 = "18.184.142.208";
       inherit sshUsers;
       hardware = ../../lib/issuer-aws.nix;