Do not run the diff GA workflow on closed PRs.

The `diff.yml` GitHub Actions workflow runs whenever a new comment is
added to a PR, to check whether the comment contains a `gogoeditdiff`
tag that asks for the production of a diff.

Unfortunately, one of the actions this workflow is dependent on does not
handle correctly the case where the PR has been closed but is still
commented on, and throws an error when that happens.

So we add an additional condition before running the workflow: the PR
has to be open.

This will prevent people from requesting a diff in a closed PR, but:

(1) this should not be something that people would need often (if the PR
has been closed, why would a diff be required);

(2) if a diff _is_ required on a closed PR, it is always possible to
re-open the PR (arguably this is what should be done anyway: if someone
is interested in the PR enough for wanting a diff and checking what are
its consequences on the ontology, that sounds like a sign that the PR is
not worthless and should be kept open);

(3) in any case, in the current situation requesting a diff on a closed
PR already does not work anyway, since the action we are dependent does
not know how to work on a closed PR.

The main interest of this change is to avoid receiving needless emails
letting us know that a workflow has failed, just because people are
commenting on a closed PR.

.github/workflows/diff.yml

@@ -10,7 +10,7 @@ env:
 # A workflow run is made up of one or more jobs that can run sequentially or in parallel
 jobs:
   branch_status:
-    if: ${{ github.event.issue.pull_request }}
+    if: ${{ github.event.issue.pull_request && github.event.issue.state == 'open' }}
     runs-on: ubuntu-latest
     permissions:
       contents: read