bench-bot: provide raw .json benchmark results as artifacts

[koute]

When you trigger the bench bot it uploads a zip file with a diff of the weights (for example, paritytech/substrate#12280 (comment)). Could we make it so that it also uploads the raw .json files with the raw benchmark measurements inside this zip file? (This requires an extra argument --json-file=$filename.json to be passed to the benchmark invocation to get those files generated.)

This would be very useful for raw performance comparisons, since using weights for this is not ideal as they are a result of a linear regression, so comparing them to measure a change in performance is tricky/not always correct.