Populate FILES variable if PR contains one file in data/

Populate `FILES` variable if PR contains one file in `data/`.

Currently often in PRs the `FILES` variable is empty when an existing PR gets updated by pressing the "Update branch" button or using Mergify, e.g., here for f578718:
https://github.com/AppImage/appimage.github.io/pull/2541

Log:
https://github.com/AppImage/appimage.github.io/runs/3149918916

In those situations, I manually have to go into the file in `data/` in the PR, make a trivial edit (e.g., add a line with a `#` in it) to trigger another test run where the changed file then is picked up in the `FILES` variable.

So we would somehow need to look at the PR and find out if file(s) in `data/` have been affected by the PR, and populate the `FILES` variable with them.

@andy5995 do you have an idea how to do that?

Without having this, using Mergify (which automates pressing the "Update branch" button) is of no actual use to us.