Use GitHub API to retrieve latest release information.

Previously we were requesting `https://volta.sh/latest-version` (which
is what the `curl` based installer workflow documented on the homepage
does). Unfortunately, we end up getting `ECONNREFUSED` from the service
we use to host https://volta.sh.

This replaces the usage of `https://volta.sh/latest-version` with
directly using GitHub's API at
`https://api.github.com/repos/volta-cli/volta/releases/latest`. The new
system is setup to retry 3 times **and** threads through the current
$GITHUB_TOKEN (which should avoid throttling issues).

Author: Robert Jackson