fix(helper): avoid an unhandled EOFError when build_image has no stdin

## Summary

`infra/helper.py build_image` prompts for a pull decision when neither `--pull`
nor `--no-pull` is provided. If the command runs without readable standard
input, the prompt raises an unhandled `EOFError` and prints a Python traceback.

This affects the plain `build_image` command shown in the reproducing and new
project guides when it is invoked by a noninteractive shell or remote runner.

## Steps to reproduce

From the OSS-Fuzz repository at current `master`:

```bash
python3 infra/helper.py build_image zlib

Start in infra/helper.py at build_image's pull-selection parser and the raw_input call described in the issue; reproduce with python3 infra/helper.py build_image zlib

Written by the indexing model from the issue text.