linux: remove assert on linux

Asserting the hostplatform for `linux` is bad, because it can't be
caught by CI. For the `linux` package itself, it doesn't make a
difference, because it also has `meta.platforms = linux` set, so this
will fail evaluation - and in a way that can nicely be caught by CI.

pkgs/os-specific/linux/kernel/generic.nix

···
     # cgit) that are needed here should be included directly in Nixpkgs as
     # files.
 
-    assert stdenv.hostPlatform.isLinux;
-
     let
       # Dirty hack to make sure that `version` & `src` have
       # `<nixpkgs/pkgs/os-specific/linux/kernel/linux-x.y.nix>` as position

pkgs/os-specific/linux/ply/default.nix

···
       mic92
       mbbx6spp
     ];
+    platforms = lib.platforms.linux;
   };
 }

pkgs/test/nixos-functions/default.nix

···
     label = "test";
   };
 in
-pkgs.recurseIntoAttrs {
-
-  nixos-test =
-    (pkgs.nixos {
-      system.nixos = dummyVersioning;
-      boot.loader.grub.enable = false;
-      fileSystems."/".device = "/dev/null";
-      system.stateVersion = lib.trivial.release;
-    }).toplevel;
-
-}
+lib.optionalAttrs (stdenv.hostPlatform.isLinux) (
+  pkgs.recurseIntoAttrs {
+    nixos-test =
+      (pkgs.nixos {
+        system.nixos = dummyVersioning;
+        boot.loader.grub.enable = false;
+        fileSystems."/".device = "/dev/null";
+        system.stateVersion = lib.trivial.release;
+      }).toplevel;
+  }
+)