switch nightly openmp build to stretch
GitLab-15.0 was released on 22 May 2022 and git.ligo.org, chat.ligo.org, containers.ligo.org, and docs.ligo.org will be updated to this release on the 14 June 2022. This release contains various new features and more importantly several breaking changes. If you are using any of the features outlined in the list of breaking changes then your workflow will encounter errors once this release is installed. If you are using any of these features and can't switch to the replacements before 14 June 2022, or have any general comments, questions, or concerns, please make this known on the following Epic.