Hrmm, resolving a literal, non-string path with ./ should resolve the absolute path, so that should work: https://nixos.org/manual/nix/stable/language/values.html#type-path
Writing an absolute path in there will still make your configuration non-reproducible if it ever is in a different directory.
Apparently when I last needed to do this certificateFiles didn’t exist, so I used builtins.readFile here: https://github.com/TLATER/dotfiles/blob/4b8a5c480ef22d086de399d093e8578a57b5b24e/nixos-config/ct-lt-02052/default.nix#L87
Mind trying that to see if it works for you? It certainly does on my end; Perhaps there’s a bug in the cert bundler specific to the -Files variant of the option.