This PR moves the size comparison action back to a separate workflow
which now uses the `pull_request_target` event. This event is triggered
at all the same times as the `pull_request` action, except that the
workflow file content comes from the target branch of the PR, and it is
run in the context of the repo owning the target branch. Practically,
this means that it will still have access to post a comment even if the
PR comes from a fork.

We don’t want the build actions to be run in a `pull_request_target`
workflow because they would get access to the secrets and be able to
perform arbitrary actions on the repository, even from fork PRs.

See the current version failing here:
https://github.com/actualbudget/actual/actions/runs/5395184895/jobs/9797388016?pr=1122