nix config to save some space

nixos/roles/nix.nix

@@ -1,5 +1,6 @@
 {
-  nix.settings = {
+  nix = {
+    settings = {
       trusted-public-keys = [
         "nerflap2-1:pDZCg0oo9PxNQxwVSQSvycw7WXTl53PGvVeZWvxuqJc="
         "gonne.mathebau.de-1:FsXFyFiBFE/JxC9MCkt/WuiXjx5dkRI9RXj0FxOQrV0="
@@ -10,7 +11,7 @@
       ];
       auto-optimise-store = true;
       fallback = true;
-    builders-use-substitus = true;
+      builders-use-substitutes = true;
     };
     gc = {
       persistent = true;
@@ -22,4 +23,5 @@
       dates = [];
       automatic = true;
     };
+  };
 }