Fix GCP helpers to use correct chmod value

[amyreese]

os.chmod() expects octal values. 777 is almost certainly not producing the intended result.

Discovered in astral-sh/ruff#22915

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Was generative AI tooling used to co-author this PR?

No

[boring-cyborg]

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[shahar1]

Nice catch!

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.