Add support for automatically selecting (at run-time) the latest available release of Juliaup

[DilumAluthge]

The idea is that the user could do e.g.

- uses: julia-actions/install-juliaup@latest
  with:
    julia-version: '1'
    internal-juliaup-version: 'latest'

And then we automatically look up the latest available release of Juliaup, and use that. So the user doesn't have to hardcode the version number of Juliaup.

We can even make latest the default value for the internal-juliaup-version input, allowing the user to do this:

- uses: julia-actions/install-juliaup@latest
  with:
    julia-version: '1'

[DilumAluthge]

I think that this would be a lot of work for relatively little payoff.

In order to make this work, we would need to use the GitHub API to fetch the list of all releases for the JuliaLang/juliaup, and then sort the release version numbers in increasing order, and then take the maximum version number.

And in order to make requests to the GitHub API, we need a GitHub token with read access.

[davidanthoff]

We could upload the latest portable archive to a stable URL on S3, and then this action could always get the Juliaup binaries from that URL, and it would always be the latest version without any need to update things here.

[DilumAluthge]

It occurs to me: we don't actually need to host the full archive somewhere. We actually only need to host a text file (on a JuliaLang domain) that contains the version number of the latest stable release of Julia. Then, in this action, we can:

1. Download that text file and extract the version number.
2. Interpolate that version number into the https://github.com/JuliaLang/juliaup/releases/download/v${juliaup_version}/juliaup-${juliaup_version}-${triplet}-portable.tar.gz URL.
3. Download the Juliaup archive from the GitHub releases URL.