Revert "singularity-tools: don't preserve store content ownership"

Use `cp -ar` instead of `cp -r` to preserve symbolic links
and other attributes whenever possible.

This reverts commit c2eb0aa56e.

pkgs/build-support/singularity-tools/default.nix

@@ -118,7 +118,7 @@ lib.makeExtensible (final: {
             mkdir -p bin ./${builtins.storeDir}
             # Loop over the line-separated paths in $layerClosure
             while IFS= read -r f; do
-              cp -r "$f" "./$f"
+              cp -ar "$f" "./$f"
             done < "$layerClosure"
 
             # TODO(@ShamrockLee):