release-cross.nix: nixCrossStatic: do not burden *-darwin builders

PR #181151 added the nixCrossStatic jobs, which cross-compile
nixStatic for (currently) mips64el and powerpc64le, so users on
those platforms can use it to bootstrap their own builds of
nixpkgs/nixos:

  https://github.com/NixOS/nixpkgs/pull/181151

However I unintentionally caused that PR to queue this job on *all*
Hydra builders, including the (heavily burdened) *-darwin builders:

  https://hydra.nixos.org/build/203846776
  https://hydra.nixos.org/build/203846780

There is no need to build these derivations with a darwin
buildPlatform.  Let's limit this task to only buildPlatform=*-linux.

pkgs/top-level/release-cross.nix

@@ -92,7 +92,7 @@ let
   # with their host distribution's versions of nix's numerous
   # build dependencies.
   nixCrossStatic = {
-    nixStatic = nativePlatforms;
+    nixStatic = platforms.linux;  # no need for buildPlatform=*-darwin
   };
 
 in