Update GitHub workflows to pick a particular Xcode #431
Description
When the builders are updated to include new Xcode versions, it can impact swiftly's CI because it pins the swift toolchain version based on the swift version file. If the macOS SDK is ever newer than the toolchain then it can cause build errors for all active PR's.
It will be good to pin the Xcode version too, so that there isn't a sudden change to the CI behaviour on the project. This can be done using the DEVELOPER_DIR environment variable.