Install `optuna-integration` in `chainer` CI

[toshihikoyanase]

Motivation

Fix CI failure due to the optuna-integration migration. The implementation of optuna.integration.ChainerPruningExtension was removed from optuna.integration and moved to the optuna-integration package in https://github.com/optuna/optuna/pull/4370.

optuna-integration is required to run the Chainer examples now.

Description of the changes

• Install optuna-integration from the git repository
• chainer/requirements.txt should be updated as well, but we cannot because the optuna-integration package has not been released in PyPI yet.

[toshihikoyanase]

@gen740 @HideakiImamura This is a follow-up PR for https://github.com/optuna/optuna/pull/4370. So, could you review it, please?