CI to ensure `from-pr` and friends are not used in easystacks, and `from-commit` is merged upstream.

[ocaisa]

We need a yaml parser CI to make sure that from-pr is not being used in easystack files. We also need to check that if from-commit is being used that the PR is merged upstream, this can be done with something like

git merge-base --is-ancestor $COMMIT_ID HEAD