build: refactor webui build and enable it in packages

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.

(cherry picked from commit f8d1bc1e)