Add hacky way to prevent Hydra from building/distributing unfree packages

If the environment variable HYDRA_DISALLOW_UNFREE is set to "1", then
evaluation of a package with license "unfree" will throw an error.
Thus such packages or any packages that depend on them will fail to
evaluate.

pkgs/stdenv/generic/default.nix

@@ -10,6 +10,8 @@ let
 
   lib = import ../../lib;
 
+  disallowUnfree = builtins.getEnv "HYDRA_DISALLOW_UNFREE" == "1";
+
   stdenvGenerator = setupScript: rec {
 
     # The stdenv that we are producing.
@@ -39,6 +41,9 @@ let
         # Add a utility function to produce derivations that use this
         # stdenv and its shell.
         mkDerivation = attrs:
+          if disallowUnfree && attrs.meta.license or "" == "unfree" then
+            throw "package ‘${attrs.name}’ has an unfree license, refusing to evaluate"
+          else
           (derivation (
             (removeAttrs attrs ["meta" "passthru" "crossAttrs"])
             // (let