Add 'alpha' version for pre-releases like juliaup?

[MilesCranmer]

juliaup has an alpha channel which is used for pre-releases. As far as I can tell there is not an equivalent here? It would be great to have this. For me, the nightly channel is way too unstable, so I end up never using it. But I do want to test on upcoming versions. (without needing to make a PR for every single pre-release, which I never do)

I think both:

• Proposal for simplifying testing of pre-releases #84
• Use juliaup to install julia #172

would solve this. Why not switch to juliaup? We get this for free then.

[MilesCranmer]

@IanButterworth do you know the status of your proposal and if anybody has worked on it?