fix(entrypoint): create dest_dir before download_distribution for nix containers

[devin-ai-integration]

Why are the changes needed?

Nix-built container images (ImageSpec(nix=True)) produce minimal filesystem layouts that may not include /root. The pyflyte register CLI defaults --destination-dir to /root, which gets baked into the task template as --dest-dir /root. When pyflyte-fast-execute runs inside a nix container, download_distribution raises:

ValueError: Destination path is required to download distribution and it should be a directory

because /root doesn't exist in the container (nix images use WorkingDir=/service).

Observed in production on: flytesnacks/staging/ax9ttpbwtd6crjxdqdfz

What changes were proposed in this pull request?

Add os.makedirs(dest_dir, exist_ok=True) in fast_execute_task_cmd before calling _download_distribution. This ensures the destination directory is created if it doesn't already exist, making the entrypoint resilient to minimal container filesystems.

How was this patch tested?

Not yet tested with a unit test. The fix was validated by tracing the exact failure path from production Loki logs through the entrypoint code. The exist_ok=True flag ensures no behavior change when the directory already exists (i.e., all non-nix containers).

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Human Review Checklist

• Confirm os.makedirs won't hit permission issues in restricted containers
• Consider whether silently creating /root could mask a misconfigured --destination-dir

---

Link to Devin run | Requested by @sammiphone6

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring