nixpkgs build system sets `RUST_LOG=` (empty value) by default.
This switches `mdBook` into warnings+ mode (instead of info+).
This causes the following tests to fail:
$ RUST_LOG= cargo test --test cli_tests
...
cli::test::mdbook_cli_can_correctly_test_a_passing_book
cli::test::mdbook_cli_detects_book_with_failing_tests
cli::build::mdbook_cli_dummy_book_generates_index_html
The change drops RUST_LOG= entry.
