pkgs: set jre_headless to jdk11_headless

this is a workaround to be able to use java 11 with the
minecraft-server module
minecraft calls for jre_headless, which is still java 8
newer java version don't ship jre, which now have to be custom built or
the jdk used

pkgs/default.nix

@@ -41,7 +41,7 @@ let
     });
 
     mattermost = callPackage ./mattermost {};
-
+    jre_headless = pkgs.jdk11_headless;
     inherit (unstable) bottom;
   };