[ci] update scripts to use swiftlang's static SDK scripts

[sebsto]

Now that the Swiftlang GitHub Workflow project added a script to test builds with generated or static SDK, we can simplify the musl part of our builds

swiftlang/github-workflows#40

[sebsto]

This is the part that needs to be updated
https://github.com/swift-server/swift-aws-lambda-runtime/blob/11bea7b2eefcae2c3005036add3c8200309d91bf/.github/workflows/pull_request.yml#L61