[PackageModel] Use correct swiftVersion for linux main targets
[PackageModel] Use correct swiftVersion for linux main targets

-- <rdar://problem/31725795> Compile LinuxMain.swift with same swift version as the test targets it contains

abertelrud/swift-package-manager,sschiau/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager