This commit introduces the ability to skip the UI build in the Makefile by
providing prebuilt UI assets, addressing the needs of users who may not have npm
installed or who do not want to go through the front-end build process.

To achieve this, we added the `PREBUILT_ASSETS_STATIC_DIR` environment variable.
If this variable is set, the Makefile will skip the UI build and related tasks,
such as bundling npm licenses. Instead, it will use the prebuilt assets from
the specified directory.

We already publish prebuilt UI assets as part of our release artifacts
(e.g., `prometheus-web-ui-3.0.0-beta.0.tar.gz`). Users can download this tarball,
extract it, and point the `PREBUILT_ASSETS_STATIC_DIR` to the extracted folder.
This reduces build complexity, especially for users who don't have a development
environment for front-end builds.

For example, you can use the command:
`make PREBUILT_ASSETS_STATIC_DIR=static build`
where `static` ...