std.Io.Writer: fix writeSliceSwap

tried to be too clever, wrote bad code

lib/std/Build/Step/Run.zig

@@ -1742,7 +1742,7 @@ fn sendMessage(file: std.fs.File, tag: std.zig.Client.Message.Tag) !void {
         .tag = tag,
         .bytes_len = 0,
     };
-    try file.writeAll(std.mem.asBytes(&header));
+    try file.writeAll(@ptrCast(&header));
 }
 
 fn sendRunTestMessage(file: std.fs.File, tag: std.zig.Client.Message.Tag, index: u32) !void {