Add convenience function l.S

Takes the same arguments as l.src but returns the necessary /bin/sh code that
creates a softlink to src and cd's to the location of the current do script.

builder.nix

@@ -21,6 +21,13 @@ let
         __functor = self: arg: resolve arg;
         src = getSrc root rcwd filter id;
         inherit pkgs cwd rcwd d1 d2;
+        # Convenience function that calculates the source tree, links it to src
+        # and cd's to the location of the current builder.
+        S = getSrc root rcwd filter (src:
+          ''
+            ln -s ${src} src
+            cd src/${rcwd}
+          '');
       };
 
       _builder = let