Run Lighthouse test only on modified pages

The current CI workflow runs the Lighthouse analysis on all rendered website pages, or at least that's what was supposed to happen. The logs seem to indicate that only a few pages --- and it's always the same pages --- are being analyzed, rather than all of the rendered HTML+CSS+JS pages on the website.

If possible, we could run the Lighthouse check only on the pages that have been modified or added in the current pull request. This might guarantee that the right pages are always tested.