fix: resolve --version returning unknown in Docker

[raajheshkannaa]

Closes #486

get_pacu_version() reads pyproject.toml by walking up from pacu/main.py.
After pip install . (as the Dockerfile does), the code runs from site-packages
where pyproject.toml doesn't exist, so it falls back to "unknown".

Fix: use importlib.metadata.version() (stdlib since Python 3.8) as the primary
source. This reads from installed package metadata, which pip install populates
correctly. The pyproject.toml fallback stays for development installs.