[font-tags] Check CSV not upstream Google Sheet

Since https://github.com/googlefonts/gftools/blob/main/Lib/gftools/scripts/font_tags.py snaggled up the main repo CI today, I had a chat with @chrissimpkins about it and I think as currently implemented this CI check may be checking at an inappropiate level - ie, the main repository shouldn't depend on tests against an upstream spreadsheet that is not in the repository. Instead, since the sheet is converted to CSV that is kept inside the repo, the CI should only check that CSV file, and updates to the CSV should be PR'd so these commits get CI help to know that something is breaking before it is merged to main branch.

The current implementation is still useful for linting the upstream sheet, so shifting it to its own separate file (https://github.com/google/fonts/pull/7058) is good, but maybe throws the baby out with the bathwater :)