support multiple versions of swift-syntax from ci jobs

[vanvoorden]

https://github.com/ordo-one/equatable/actions/runs/16604287942/job/46971962407?pr=24#step:8:12

Our current build-macos job builds and tests against one version of swift-syntax: the latest version. But our macro package actually supports four versions:

• 600.0.0
• 600.0.1
• 601.0.0
• 601.0.1

If an engineer updating our macro package builds and tests locally against 601.0.1 they might accidentally make a change that is not supported on previous versions.

For extra protection we can update the build-macos job to switch against multiple versions of swift-syntax.

Some examples of how this works:

• Empire
• Swift-CowBox
• swift-macro-compatibility-check