Support a build metadata file to be displayed in the about dialog

To aid in build debugging, it would be useful to include a metadata file that build pipelines could add information to that could later be displayed in the setup dialog.

Proposed implementation:
- Create a file at `angrmanagement.build_metadata` with default text "Custom git build". Add to gitignore so local changes in the future aren't tracked
- Ensure file is included in sdist/wheel and pyinstaller
- Edit about dialog to display the text, possibly hidden by default under a "Show build information" expanding box
- Update build pipelines to insert git information into this file during the build, including nightlies, angr release, and checrs (probably as a separate task from the first three bullets)