Fix relative path issue with `minify`

[adamscott]

The minify generation had the following error on the github action:

Minifying assets
ERROR: stat .//home/runner/work/godot-website/godot-website/_site: no such file or directory
                    done in 8.002 seconds.

This now uses ruby pathname library to get a relative path.

Follow-up of #863 Fixes #860

[winston-yallow]

Thanks!