[Windows] Don't try to detect shell. (#253)

Author: damuellen

Sources/ArgumentParser/Completions/CompletionsGenerator.swift

@@ -42,10 +42,14 @@ public struct CompletionShell: RawRepresentable, Hashable, CaseIterable {
 
   /// Returns an instance representing the current shell, if recognized.
   public static func autodetected() -> CompletionShell? {
+#if os(Windows)
+    return nil
+#else
     // FIXME: This retrieves the user's preferred shell, not necessarily the one currently in use.
     guard let shellVar = getenv("SHELL") else { return nil }
     let shellParts = String(cString: shellVar).split(separator: "/")
     return CompletionShell(rawValue: String(shellParts.last ?? ""))
+#endif
   }
 
   /// An array of all supported shells for completion scripts.