As a software developer contributing to SwiftPM's SwiftBuild Integration work, I want PR builds that ensure we can progressively transition to it as the default build system

Acceptance Criteria

• The existing pipelines are not impacted
• A new CI build exists for each of the following platforms: Windows, MacOS, Linux
• For each of the new builds
  • Uses the latest stable Swift nightly main toolchain on
  • Initially, the build is not marked mandatory/required in GitHub
  • Build invokes the following command ./Utilities/build-using-self --build-system swiftbuild. On Windows, the command must be prefix with python3 executable.