Store failure screenshots as test artifacts in CI

When system tests fail, Capybara automatically takes a screenshot of the
page to help developers debug the context of the failure. This was
introduced in 6ca32a1.

This commit configures the GitHub Actions CI job to store those
screenshots as test artifacts. This makes them available to developers
following a failed build.

Inspired by DFE-Digital/apply-for-teacher-training#8848

.github/workflows/deploy.yml

@@ -97,3 +97,10 @@ jobs:
           bundle exec rake db:create
           bundle exec rails db:schema:load
           bundle exec rspec --format progress
+      - name: Upload failure screenshots
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: failure-screenshots
+          path: tmp/capybara/
+          if-no-files-found: ignore