Support env-variable FROGMOUT_CFG_FILE to set the config-location

[woutervh]

The config-file is currently hardcoded.

def config_file() -> Path:
    """Get the path to the configuration file.

    Returns:
        The path to the configuration file.

    Note:
        As a side-effect, the configuration directory will be created if it
        does not exist.
    """
    (config_dir := xdg_config_home() / ORGANISATION_NAME / PACKAGE_NAME).mkdir(
        parents=True, exist_ok=True
    )
    return config_dir / "configuration.json"

This is not virtualenv-friendly.

virtualenv allows to install multiple python-apps in parallel,
but it's problematic when they all want to use the same -configfile in $HOME

pleasa support setting this location by env-var:

> export FROGMOUT_CFG_FILE = ....