CI: skip package uploading and deployment steps on non-release revisions
Traditionally CI has uploaded release artifacts and packaged a “release” for
every single commit on the main branch. These inter-release packages were
intended for testing and internal deployment. As far as we aware these are no
longer used; all Graphviz developers build from source and do not rely on
Gitlab’s generic package repository.¹
This change rearranges deployment steps to skip uploading artifacts and
packaging if the current commit is not a proper release. This should slightly
accelerate CI and reduce Graphviz storage requirements, which are currently at
>800GB on Gitlab.