s3: make SetModTime work for GLACIER while syncing - Fixes #3224

Before this change rclone would fail with

    Failed to set modification time: InvalidObjectState: Operation is not valid for the source object's storage class

when attempting to set the modification time of an object in GLACIER.

After this change rclone will re-upload the object as part of a sync if it needs to change the modification time.

See: https://forum.rclone.org/t/suspected-bug-in-s3-or-compatible-sync-logic-to-glacier/10187
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