Add FreeBSD support

Following 7aeadb06ee

src/Command.zig

@@ -323,7 +323,7 @@ fn setupFd(src: File.Handle, target: i32) !void {
                 }
             }
         },
-        .ios, .macos => {
+        .freebsd, .ios, .macos => {
             // Mac doesn't support dup3 so we use dup2. We purposely clear
             // CLO_ON_EXEC for this fd.
             const flags = try posix.fcntl(src, posix.F.GETFD, 0);