fix: resolve the DEFAULT_PYTHON to the actual absolute path

[huxuan]

• I have added a news fragment under changelog.d/ (if the patch affects the end users)

Summary of changes

Fix #965

Use the resolved absolute path for DEFAULT_PYTHON can change the default python of pipx-managed pipx from /usr/local/pipx/venvs/pipx/bin/python to /usr/bin/python3.10. In this way, pipx reinstall-all will work for pipx-managed pipx.

Test plan

Tested by running

# command(s) to exercise these changes
pipx install pipx
pipx reinstall-all