Allow custom env variables in Package.swift

Currently if you try something like this in your Package.swift

if let _ = String.fromCString(getenv("TEST")) {
    let package = Package(name: "getenv_was_true")
} else {
    let package = Package(name: "getenv_was_false")
}

it uses the else path whether TEST is set or not which I assume is due to the explicit environment parameter here:
https://github.com/apple/swift-package-manager/blob/64f1b2f5bd30d1dabb59a63181243becdd8797bb/Sources/dep/Manifest.swift#L158

I wrote a sample project for the issue:
https://github.com/Memorion/getenvdemo