Improve `make validate` in CI

Add a check to CI to flag any unexpected changes in the output of make validate

assigned to @bbb

Relevant upstream issue: https://github.com/coq/coq/issues/13324

mentioned in commit 0c41e555

mentioned in commit 59641013

mentioned in merge request !154 (merged)

mentioned in commit e858b39c

closed with commit e858b39c