--pip-args does not work with args that start with "--"

[gwerbin]

Describe the bug

Passing a flag to --pip-args that starts with -- does not work properly. If you write --pip-args --foo, Pipx separates this into two separate options, --pip-args and --foo, then exits with an error code because --pip-args now is missing its argument.

How to reproduce

pipx install --pip-args --pre hy

Expected behavior

Pipx should pass the --pre option to Pip when installing Hy.

[uranusjr]

pipx install --pip-args=--pre=py.

python/cpython#53580