Add "warning" verbosity level

This ensures that "nix" shows warnings. Previously these were hidden
because they were at "info" level.

(cherry picked from commit 615a9d031d)

src/nix/main.cc

@@ -93,7 +93,7 @@ void mainWrapped(int argc, char * * argv)
         if (legacy) return legacy(argc, argv);
     }
 
-    verbosity = lvlError;
+    verbosity = lvlWarn;
     settings.verboseBuild = false;
 
     NixArgs args;