make-bootstrap-tools*: fixup after #16406

Our coreutils now uses single-binary-build mode where, by default,
simple shebang scripts are used for all the binaries. That doesn't work
e.g. with the Linux unpacker which only handles standard binaries and
symlinks. Let's use the symlinked mode instead for boostrapping.
This does NOT change any stdenv hashes.

I only tested the case most important to me:
$ nix-build pkgs/top-level/release.nix -A stdenvBootstrapTools.x86_64-linux.test