Fix GitDagBundle submodule clone not using SSH settings from connection

[shunsuke-sugita]

What

GitDagBundle now wraps git submodule sync / submodule update with GIT_SSH_COMMAND from the Git connection (via GitPython custom_environment), using the worktree Repo (self.repo.git), not the bare mirror.

Why

configure_hook_env() populates hook.env, and bare clone passes it with clone_from(..., env=...). Submodule commands are separate Git subprocesses and did not receive GIT_SSH_COMMAND, so SSH URLs in .gitmodules could fail (host key / identity) even when the main repo clone worked.

Example failure (before this fix)

Submodule update could fail with errors such as:

2026-04-08T03:39:43.516370889Z Cloning into '{submodul_path}'...
2026-04-08T03:39:43.516372014Z Host key verification failed.
2026-04-08T03:39:43.516373389Z fatal: Could not read from remote repository.
2026-04-08T03:39:43.516374555Z
2026-04-08T03:39:43.516375430Z Please make sure you have the correct access rights
2026-04-08T03:39:43.516376389Z and the repository exists.
2026-04-08T03:39:43.516377430Z fatal: clone of '{submodule}' into submodule path 
'/tmp/airflow/dag_bundles/{submodule_path}s' failed
2026-04-08T03:39:43.516378680Z Failed to clone '{submodule}' a second time, aborting'
2026-04-08T03:39:43.518691472Z RuntimeError: Error pulling submodule from repository

Notes

• Aligns with the existing pattern in _fetch_bare_repo for origin.fetch on the bare repo.
• Unit tests still assert that submodule sync/update are invoked when submodules=True; no behavior change to other bundle modes.

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