Undo env vars set in the backend and write workers when Loki AWS SDK issue is fixed

[sinapah]

See also: grafana/loki#21624

When the Lokitool restarts to successfully sync to all S3 backends (probably in major 3.8 and up), we should undo the setting of these env vars in the backend workers.

if "backend" in worker.roles:
            env.update(
                {
                    "AWS_REQUEST_CHECKSUM_CALCULATION": "when_required",
                    "AWS_REQUEST_CHECKSUM_VALIDATION": "when_required",
                }
            )

[sinapah]

Update: we now have a similar problem with the write units that try to sync chunks to S3 storage. We should add the same env vars to the write units' pebble env and undo when the upstream problem is fixed (most likely in Loki 3.8).