Release-page: Fix missing [BUCKET_PATH] in component release page

What

This merge request introduces the BUCKET_PATH environment variable for production components release pages.

Why

This is necessary for the production Octez release page, so that links are correctly definied.

How

This is done and used in <COMPONENT>/scripts/releases/publish_release_page the same way as for scripts/releases/publish_release_page. We split S3_BUCKET into S3_BUCKET and BUCKET_PATH so that BUCKET_PATH can be used independently.

Before this merge request:

• S3_BUCKET is definied as the URL to upload assets in the release page. For instance, in production, it is site.prod.octez.tezos.com/releases.

As a result, we can't use /releases on its own, for instance to define links.

After this merge request:

• S3_BUCKET is defined as the address of the bucket. For instance, in production, it is site.prod.octez.tezos.com.
• BUCKET_PATH is defined as the path for the release page in this bucket. For instance, in production, it is /releases.

We use ${S3_BUCKET}${BUCKET_PATH}/ when we need the full URL, for instance when uploading the assets, in the bucket. We use ${BUCKET_PATH} when we need the path in the site, for instance for links to other pages.

Manually testing the MR

Tested in https://gitlab.com/tezos/tezos/-/pipelines/2202162741.

Result in https://octez.tezos.com/releases/

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR