Fix Python 3.8 CI environment setup

[koriyoshi2041]

Summary

• pin the virtualenv seeder to the last line that still provisions Python 3.8 environments
• run checks when workflow files change, so CI fixes validate their own configuration

Why

The Python 3.8 job currently stops before dependency installation because Poetry asks the latest virtualenv seeder to create a 3.8 environment. That seeder no longer bundles pip/setuptools for Python 3.8. Pinning virtualenv below 20.36 keeps the existing Poetry workflow and restores a seeder that supports the repository's oldest tested interpreter.

Verification

• workflow YAML parses successfully
• the first hosted run reproduced a separate global-environment conflict when virtualenv creation was disabled; the follow-up keeps isolation and pins only the incompatible seeder
• git diff --check

The updated workflow is the behavioral test for this change; removing .github/** from paths-ignore allows it to run on this PR.

[koriyoshi2041]

This CI-only change is fully green and is the remaining setup dependency for #105. Happy to adjust it if the project would prefer a different Python 3.8 compatibility path.