Add skip_pip_install to build.py (#25982)
### Description
<!-- Describe your changes. -->
- Introduce a new command-line flag `--skip_pip_install`, similar to the
existing `--skip_submodule_sync`, to allow users to prevent `build.py`
from modifying their current Python environment.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
- This is useful for scenarios where dependencies are already managed
externally or when users want to avoid unintended changes during the
build process.
