Use iterateAssumeFirstIteration in Walker.next to avoid unnecessary lseek calls

Since we are opening each directory for iteration, we know that we don't need to reset the cursor's directory before iterating. Using `iterateAssumeFirstIteration` skips the cursor resetting which eliminates an `lseek` syscall for every directory opened on non-Windows platforms.

This doesn't seem to actually matter much for performance (1.01 ± 0.02 times faster when walking /home/ on my system) but avoiding unnecessary syscalls is always nice anyway.

lib/std/fs.zig

@@ -972,7 +972,7 @@ pub const IterableDir = struct {
                         {
                             errdefer new_dir.close();
                             try self.stack.append(StackItem{
-                                .iter = new_dir.iterate(),
+                                .iter = new_dir.iterateAssumeFirstIteration(),
                                 .dirname_len = self.name_buffer.items.len,
                             });
                             top = &self.stack.items[self.stack.items.len - 1];