Fix program name in generated manual page

[jbrobst]

• I have added an entry to docs/changelog.md

Summary of changes

243c631 caused python scripts/generate_man.py (e.g. via nox -s build_man) to produce a manual page with generate_man.py as the program name. Fix this by changing sys.argv[0] to pipx before creating the parser.

Test plan

AFAICT scripts/generate_man.py is not currently tested.

[dukecat0]

Thanks!