lash: delay assert behind eval of `meta`

This allows CI to evaluate `meta.platforms` to see that this package is
not supported on darwin. Working around the assert before that requires
much more hackery.

I'm not 100% sure whether the assert is needed, but it doesn't hurt to
keep it in this place.

pkgs/by-name/la/lash/package.nix

···
   readline,
 }:
 
-assert libuuid != null;
-
 stdenv.mkDerivation rec {
   pname = "lash";
   version = "0.5.4";
···
     libxml2
     readline
   ];
-  propagatedBuildInputs = [ libuuid ];
+  propagatedBuildInputs =
+    assert libuuid != null;
+    [ libuuid ];
   NIX_LDFLAGS = "-lm -lpthread -luuid";
 
   postInstall = ''