fix: Avoid Python import race condition for Kubernetes executor stacklets

[sbernauer]

Description

Part of #732

I noticed this while looking into SUP-396. We already have code in place to prevent Python import race-conditions by pre-cloning the git repo contents before starting the Python processes.
However, that was only done for celery-based stacklets:

    // If the DAG is modularized we may encounter a timing issue whereby the celery worker
    // has started *before* all modules referenced by the DAG have been fetched by gitsync
    // and registered. This will result in ModuleNotFoundError errors. This can be avoided
    // by running a one-off git-sync process in an init-container so that all DAG
    // dependencies are fully loaded. The sidecar git-sync is then used for regular updates.
    let use_git_sync_init_containers = matches!(executor, AirflowExecutor::CeleryExecutors { .. });

While being still on SDP 25.3 (so before our platform fix), the customer in SUP-396 correctly reported that they are using Kubernetes executors, which have the same problem [for anything except executors].

This PR fixes the mechanism to also run on Kubernetes executor-based stacklets

Definition of Done Checklist

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Author

• Changes are OpenShift compatible
• CRD changes approved
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Reviewer

• Code contains useful comments
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• (Integration-)Test cases added
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• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

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Release notes

Fixed

Avoid Python import race conditions by pre-cloning the git repo not only for Celery-based stacklets, but also for Kubernetes executor-based stacklets

[sweb]

Do we need to change the existing gitsync test to catch this? If I see correctly, right now we just look for the config map - I am not sure whether that is enough to catch e.g. slow network. Does it make sense to add a check there to the statefulset or something like that, that we actually load the dags?

[sweb]

lgtm - I'd still say the real approval has to be done by Andrew

[adwk67]

LGTM - thanks!