nix: Add option to use alejandra for formatting

Applying patch from nix-community/rnix-lsp#89
directly on top of current rnix-lsp and providing UseAlejandra command
to switch to in when Vim is already running.

nix/nix.nix

@@ -1,4 +1,27 @@
 { config, pkgs, ... }:
+let
+  rnix-lsp-alejandra =
+    let
+      src = pkgs.fetchFromGitHub {
+        owner = "nix-community";
+        repo = "rnix-lsp";
+        # https://github.com/nix-community/rnix-lsp/pull/89
+        rev = "9189b50b34285b2a9de36a439f6c990fd283c9c7";
+        sha256 = "sha256-ZnUtvwkcz7QlAiqQxhI4qVUhtVR+thLhG3wQlle7oZg=";
+      };
+    in
+    pkgs.rnix-lsp.overrideAttrs (old: {
+      inherit src;
+      cargoDeps = old.cargoDeps.overrideAttrs (old: {
+        inherit src;
+        outputHash = "sha256-nT+tvOYiqjxLL+bHH/6PMPJbZDg5znaHEJol4LkCBCA=";
+      });
+      cargoBuildFlags = [ "--no-default-features" "--features" "alejandra" ];
+      postInstall = ''
+        mv $out/bin/rnix-lsp $out/bin/rnix-lsp-alejandra
+      '';
+    });
+in
 
 {
   programs.direnv = {
@@ -14,13 +37,15 @@
   programs.neovim = {
     extraConfig = ''
       let g:LanguageClient_serverCommands['nix'] = ['rnix-lsp']
+      command! UseAlejandra call LanguageClient#shutdown()|let g:LanguageClient_serverCommands['nix'] = ['rnix-lsp-alejandra']|call LanguageClient#startServer()
     '';
   };
 
   home.packages = with pkgs; [
     lorri
     nixUnstable
     rnix-lsp
+    rnix-lsp-alejandra
   ];
 
   launchd = {