ruby: don't reference groff if not being used (not used by default)

Fixes cross build by removing false dependency.

If this is needed again, be careful about *which* "groff" is needed.
(which in terms of build vs host)

pkgs/development/interpreters/ruby/default.nix

@@ -73,7 +73,7 @@ let
             unpackdir rubySrc;
 
         # Have `configure' avoid `/usr/bin/nroff' in non-chroot builds.
-        NROFF = "${groff}/bin/nroff";
+        NROFF = if docSupport then "${groff}/bin/nroff" else null;
 
         nativeBuildInputs =
              ops useRailsExpress [ autoreconfHook bison ]