user: build use unstable packages for nil

cannot be built on stable because it requires rustc 1.66 or newer, while the currently active rustc version is 1.64.0

home/modules/dev_tools.nix

@@ -43,7 +43,7 @@ in
     nickel
 
     nixpkgs-fmt # nix formatter
-    nil # nix lsp server
+    unstable.nil # nix lsp server
 
     docker-compose
   ];