purePackages.gen: Don't build on Hydra (#29763)

Until https://github.com/agraef/pure-lang/issues/8 is resolved, this
package can't be compiled. This patch turns off Hydra builds of this
package as part of #28643 (zero Hydra failures)

pkgs/development/pure-modules/gen/default.nix

···
     homepage = http://puredocs.bitbucket.org/pure-gen.html;
     license = stdenv.lib.licenses.free;
     platforms = stdenv.lib.platforms.linux;
+    hydraPlatforms = [];
     maintainers = with stdenv.lib.maintainers; [ asppsa ];
   };
 }