make-disk-image: use filterSource instead of cleanSource

cleanSource does not appear to work correctly in this case. The path
does not get coerced to a string, resulting in a dangling symlink
produced in channel.nix.  Not sure why, but this
seems to fix it.

Fixes #51025.

/cc @elvishjericco

nixos/lib/make-disk-image.nix

@@ -84,7 +84,7 @@ let format' = format; in let
   # FIXME: merge with channel.nix / make-channel.nix.
   channelSources = pkgs.runCommand "nixos-${config.system.nixos.version}" {} ''
     mkdir -p $out
-    cp -prd ${nixpkgs} $out/nixos
+    cp -prd ${nixpkgs.outPath} $out/nixos
     chmod -R u+w $out/nixos
     if [ ! -e $out/nixos/nixpkgs ]; then
       ln -s . $out/nixos/nixpkgs