treewide: remove overly broad with cfg

Makes it really hard to follow references and we were being explicit in
most places already anyway.

mail-server/environment.nix

@@ -25,17 +25,15 @@ let
   cfg = config.mailserver;
 in
 {
-  config =
-    with cfg;
-    lib.mkIf enable {
-      environment.systemPackages =
-        with pkgs;
-        [
-          dovecot
-          openssh
-          postfix
-          rspamd
-        ]
-        ++ (if certificateScheme == "selfsigned" then [ openssl ] else [ ]);
-    };
+  config = lib.mkIf cfg.enable {
+    environment.systemPackages =
+      with pkgs;
+      [
+        dovecot
+        openssh
+        postfix
+        rspamd
+      ]
+      ++ (if cfg.certificateScheme == "selfsigned" then [ openssl ] else [ ]);
+  };
 }