fix(storage/chunk/client/aws): have GetObject check for canceled context (backport k223)

[loki-gh-app]

Backport 5f325aa from #14420

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What this PR does / why we need it:

S3ObjectClient.GetObject incorrectly returned nil, 0, nil when the provided context is already canceled.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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