[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

swift-aws-lambda-runtime/.github/workflows/pull_request.yml

Line 61 in 11bea7b

musl: