sage: don't test the downgraded R

Fails transiently. See https://github.com/NixOS/nixpkgs/pull/49249. The
failures look harmless and are fixed with R 3.5 which will be used in
sage 8.5.

pkgs/applications/science/math/sage/default.nix

@@ -129,6 +129,7 @@ let
     # https://trac.sagemath.org/ticket/25674
     R = pkgs.R.overrideAttrs (attrs: rec {
       name = "R-3.4.4";
+      doCheck = false;
       src = fetchurl {
         url = "http://cran.r-project.org/src/base/R-3/${name}.tar.gz";
         sha256 = "0dq3jsnwsb5j3fhl0wi3p5ycv8avf8s5j1y4ap3d2mkjmcppvsdk";