nixos/lib/make-ext4-fs: fix after mkfs.ext4 refactoring

In 9ac1ab10c9 this library function was
refactored to use mkfs.ext4 instead of cptofs. There are two problems:

If populateImageCommands would create no files (which is fine), a cp
invocation would fail due to missing source arguments.

Another problem is that mkfs.ext4 relies on fakeroot to have sane
uid/gids in the generated filesystem image. This currently doesn't
work for cross compiling.

nixos/lib/make-ext4-fs.nix

@@ -46,7 +46,10 @@ pkgs.stdenv.mkDerivation {
       (
         GLOBIGNORE=".:.."
         shopt -u dotglob
-        cp -a --reflink=auto ./files/* -t ./rootImage/
+
+        for f in ./files/*; do
+            cp -a --reflink=auto -t ./rootImage/ "$f"
+        done
       )
 
       # Also include a manifest of the closures in a format suitable for nix-store --load-db