Add support for running builds using macOS OSS Toolchain #240
Description
As a software developer contributing to Swift project, I want to be able to run GitHub Actions using the macOS OSS toolchain so I can verify my changes against a macOS toolchain that contains my changes.
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The current macOS GitHub Actions run against a pre-installed Xcode. This has a released version of Swift toolchain. There are time where the changes I want to validate are in an unreleased Swift toolchain version (e.g.: main nightly), but there is currently no way to execute GitHub Actions against an unreleased version of the Swift Toolchain.
This issue is to track adding GitHub Actions support for running against a macOS OSS Toolchain.
One use case: Swift For Embedded does not require Xcode.