Automate testing of changes to distribution-scripts

[straight-shoota]

After every change to distribution-scripts, that affect the release packaging process, we need to test if they work in the CI workflow of https://github.com/crystal-lang/crystal/.

This could be automated. For every PR, a corresponding test branch needs to be created at https://github.com/crystal-lang/crystal/. The branch name must start with ci/ to trigger a maintenance build. This could just be prefixed to the branch name in https://github.com/crystal-lang/distribution-scripts.
That test branch must contain a commit which updates the commit SHA in https://github.com/crystal-lang/crystal/blob/5663eb00613114a7d6a784428f1d0af030a2aa09/.circleci/config.yml#L147 to the HEAD of the distribution-scripts branch. For this purpose we should probably extract that reference to an easily modifyable value.
For every subsequent commit to the distribution-scripts branch, there should be a respective update to the test branch.
A comment should be posted to the PR with a link to the test branch and the corresponding workflow run at circleCI.