The existing `livecheck` block for `todesk` checks an upstream release
notes page but the response body only contains a `<script>` element
when checked with curl, so it gives an `Unable to get versions` error.
This seems to be true regardless of headers (e.g., user agent, etc.).

This updates the `livecheck` block to check the download page and
match the version from the pkg URL (found in some JavaScript code).