feat: add sizeLabel GITHUB_OUTPUT for further reuse in other jobs

[mat3e]

I wanted my GitHub Workflow to comment in the PR (using https://github.com/thollander/actions-comment-pull-request) in case its size is too big.

I tried using github.event.pull_request.labels.*.name, but it contains a previous PR state (without a newly added size label), so I needed to be more proactive and added writing to GITHUB_OUTPUT file right from the Action script to capture the assigned sizeLabel.

[mat3e]

@pascalgn wdyt? :)