framenix: move nix build dir to tmpfs

hosts/framenix/storage.nix

@@ -39,6 +39,17 @@ in
       fsType = "vfat";
       options = [ "discard" ];
     };
+    # nix/ lix build directory
+    # lix:
+    # > If you plan to use a tmpfs, we **strongly recommend** to set `mode=0755` as a mount option for that `tmpfs`, otherwise you are effectively reverting this mitigation.
+    "/nix/var/nix/builds" = {
+      device = "tmpfs";
+      fsType = "tmpfs";
+      options = [
+        "mode=0755"
+        "size=75%"
+      ];
+    };
   };
 
   services.fstrim.enable = true;