Fix metadata reminder to work with forks (#1742)

This PR updates the reminder to modify the `dateOfLastModification` so
it will work when PRs are opened from forks. One consequence is that the
check for changed files must now take place within the body of the
script by calling the Github CLI directly, because the action's paths
check doesn't work as expected when the workflow trigger is
`pull_request_target`.

**Potential risks**:
The new event trigger means the associated GITHUB token will have
elevated privileges, but the scope is limited to posting a comment on
the PR, and no code is checked out and no secrets are involved, so the
risk is minimal.

Author: doctorperceptron

.github/workflows/update-metadata-reminder.yml

@@ -1,9 +1,7 @@
 name: Reminder to update demo metadata
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, reopened]
-    paths:
-      - demonstrations_v2/**
 
 jobs:
   reminder:
@@ -12,6 +10,13 @@ jobs:
     - uses: actions/github-script@v7
       with:
         script: |
+          const files = await github.paginate(
+            github.rest.pulls.listFiles,
+            { owner: context.repo.owner, repo: context.repo.repo, pull_number: context.issue.number }
+          );
+          const touchesDemos = files.some(f => f.filename.startsWith('demonstrations_v2/'));
+          if (!touchesDemos) return;
+
           github.rest.issues.createComment({
             issue_number: context.issue.number,
             owner: context.repo.owner,