pkgsLinux: set crossSystem instead of localSystem

As the doc implies, we should be using crossSystem to build *for* the
Linux platform natively instead of hoping that the system has a remote
builder or substituer with the right set of packages.

While this could be considered a breaking change, pkgsLinux attribute is
used exclusively for nixosTests and is a relatively new addition so
there shouldn’t be many external users that would be affected by this
change.

pkgs/top-level/stage.nix

@@ -249,7 +249,7 @@ let
       if stdenv.hostPlatform.isLinux
       then self
       else nixpkgsFun {
-        localSystem = lib.systems.elaborate "${stdenv.hostPlatform.parsed.cpu.name}-linux";
+        crossSystem = lib.systems.elaborate "${stdenv.hostPlatform.parsed.cpu.name}-linux";
       };
 
     # Extend the package set with zero or more overlays. This preserves