nitter: compile markdown

This currently compiles about.html, served at /about.

This step was added upstream in
8c2e0c66e271673a9dfe0bad22b523e4d2799049.

pkgs/servers/nitter/default.nix

···
 
   postBuild = ''
     nim c --hint[Processing]:off -r tools/gencss
+    nim c --hint[Processing]:off -r tools/rendermd
   '';
 
   postInstall = ''