This would allow us to identify issues with upcoming Swift releases rather than after they're available via Xcode and our users hit issues.

Snapshots are available here: https://swift.org/download/#snapshots

We could use the popular swiftenv tool to facilitate installing these snapshots, but doing it "manually" isn't difficult either.

Ideally this would be set up as a non-fallable CI job (e.g. it could always report green, but post a comment on the PR if it failed).