Web UI feature is already enabled in CI `test` stage. This patch adds it
to the `pack` stage.

To do that, a path to a webui buldle can be passed explicitly, and it
does in CI which now builds it in a separate stage.

In default Rust features `webui` remains disabled by default to keep
minimal development environment truly minimal.

The most obvious benefit is link checking. This is especially useful with rust-analyzer, because it allows to jump to definition of the item referenced in the doc comment.

The patch also makes CI to use `make lint` to avoid duplication.

Also fix some fresh clippy warnings

To run the test you need xk6 installed
(https://github.com/tarantool/xk6-tarantool).

Pytest xdist significantly reduce integration tests time
by splitting test cases into parallel child process.
Enable this feature by `-n` or `--numprocesses`,
set to 'auto' for optimal hardware resource usage.