FreeBSD nix port

Fails to build with mdbook 0.5.0:

ERROR invalid key `substitute-search`

and with mdbook 0.5.1:
ERROR The "substitute" preprocessor exited unsuccessfully with exit status: 1 status

Any clues about this errors?
Thanks

See also:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291079
https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-051