fix: use errors.As to detect TaskfileNotFoundError in getRootNode

[railgun-0402]

Summary

• Fix regression where task shows No Taskfile found at "" instead of the full error message with path and --init suggestion

Problem

When running task in a directory without a Taskfile, NewFileNode() in node_file.go wraps os.ErrNotExist into a TaskfileNotFoundError before returning. In getRootNode() (setup.go), the
error check uses os.IsNotExist(err), which fails to match the already-wrapped error. As a result, the enriched error (with the correct path, Walk: true, and AskInit: true) is never constructed,
and the incomplete TaskfileNotFoundError from NewFileNode() is returned as-is.

Fix

Replace os.IsNotExist(err) with errors.As(err, &taskfileNotFoundErr) so that the wrapped TaskfileNotFoundError is properly detected.

Before: task: No Taskfile found at ""
After: task: No Taskfile found at "/path/to/dir" (or any of the parent directories). Run task --init to create a new Taskfile.

Closes #2581