fix: use .format instead of os.path.join for Processing S3 paths.

[nadiaya]

S3 paths always use forward slashes ("/") regardless of the OS,
so using an os.path.join will cause issues when the user is on Windows.

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[knakad]

Minor: Rephrase commit message to read: "fix: use .format instead of os.path.join for Processing S3 paths"
Best to state what we're now doing instead of only what we're not doing.

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