pg-compatability: add commit sha to test comment

Add the `commit sha` that the tests run against for better clarity.
This could be retrieved as a `bash` command because it is not available in the `github context` when using `workflow_dispatch` or `issue_comment`.

Start by locating the pg-compatability workflow and the step that posts the test comment; inspect how workflow_dispatch and issue_comment expose the GitHub context. Done means the comment identifies the commit SHA used by the tests in both triggering scenarios.

Written by the indexing model from the issue text.