Fix build issue with Swift 5.7, 5.8, 5.9

[jrothwell]

For an unclear reason, builds are currently failing for Swift 5.7, 5.8, and 5.9 on the Swift Package Index for v1.2.0 and above.

• Determine why
• Stop it failing
• Add something to the pipeline to stop a build failure of this type making it into the release again - maybe by building a sample project.