Move away from or fork swift-actions/setup-swift GH action

[finestructure]

The action doesn't update to new Swift versions automatically. This slows us down unnecessarily when adopting new Swift versions.

https://github.com/SwiftPackageIndex/PackageList-Validator/actions/runs/4553151705

This affects two of the steps in #2306 : the nightly job in Update PackageList and Update PackageList-Validator.

[finestructure]

Change proposed upstream: swift-actions/setup-swift#549

[finestructure]

Also fixed in https://github.com/SwiftPackageIndex/PackageList-Validator/pull/41/files