github: make the LFS readable for repos that are public

[vit-zikmund]

This has come out of a brainstorm in https://github.com/datopian/giftless/issues/166#issuecomment-2265785088 and as the issue's original topic will be covered by PR #175, this is being separated out not to get lost in a closed issue.

For the record, checking if a repo is readable would be done with the Get a repository API call and checking the visibility field for public or internal. public is simply readable to the world, internal only to members of GitHub Enterprise organization. I assume the latter wouldn't be visible at all for a user outside of the enterprise organization, so a 40x response is a good marker the repo is not readable.

This will now need to be thought through for both the personal access tokens and app installation tokens.