nixos/portunus: fix specification of client secret

Environment variables are only expanded in *Env variables.

Ref dexidp/dex#1664

nixos/modules/services/misc/portunus.nix

@@ -214,7 +214,7 @@ in
           inherit (client) id;
           redirectURIs = [ client.callbackURL ];
           name = "OIDC for ${client.id}";
-          secret = "$DEX_CLIENT_${client.id}";
+          secretEnv = "DEX_CLIENT_${client.id}";
         });
       };
     };