.swift-version file should only be used when specified with a flag and swiftly run #504
Description
Related to #421 - swift build/swift run et al should default to the currently installed/set Swift version. There should be a separate flag to use via swiftly run swift build --use-swift-version.
This allows the swift version file to be used if required but stops people hitting the issue described in #421, which can be extremely frustrating and confusing to newcomers and experienced devs alike