schedulingPriority should be an int, fix check-meta type and in-tree use

pkgs/build-support/release/source-tarball.nix

···
 
       # Tarball builds are generally important, so give them a high
       # default priority.
-      schedulingPriority = "200";
+      schedulingPriority = 200;
     };
   }
 

pkgs/stdenv/generic/check-meta.nix

···
     position = str;
     repositories = attrsOf str;
     isBuildPythonPackage = platforms;
-    schedulingPriority = str;
+    schedulingPriority = int;
     downloadURLRegexp = str;
     isFcitxEngine = bool;
     isIbusEngine = bool;