Fixes #3862 requested exposing the built sdist/wheel path for use in commands. The TOX_PACKAGE environment variable already provides this — it is set in RunToxEnv.environment_variables after the package is built during _setup_pkg(), and is available by the time commands are resolved in run_commands(). However, this wasn't documented as a recipe for the common use case of running post-build checks like twine check or check-wheel-contents against the built artifact

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• [] ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation