Drop setup-swift (for Linux, which is the prod platform)

[finestructure]

This drops our setup-swift dependency. While it's a nice tool to set up Swift versions across platforms, it doesn't support pre-release versions (nightlies) and therefore makes it harder for us to add them overall before Swift versions are final.

This change itself does not yet switch to a nightly, that's going to be a separate PR.

[finestructure]

I've disabled the macOS test, because we're running validation on Linux and this platform test was just around because it was easy to do via setup-swift. Since we're now dropping it, maintaining the macOS test would be complicated.

[finestructure]

Going back to draft for now, because in order for this to work end-to-end, we also need to eliminate setup-swift from package list validation. Which is a bit tricky, because running swift in a docker container while managing output files can be problematic and we need to test this first.

[daveverwer]

Dropping a tick so you can merge, but understood about docker and testing first.

[finestructure]

Ok, this is actually working very seamlessly when running it in containers: https://github.com/SwiftPackageIndex/PackageList/actions/runs/5212521427/jobs/9406276077 (job succeeded, just cancelled out of it because it'd run for ages)