Switch store for randomness retrieval

[austinabell]

Summary of changes
Changes introduced in this pull request:

• So the randomness retrieval was using the runtime store, which is the gas charging blockstore lol
  • Side note following logic of gas charges, I don't see why this would be excluded from being charged, but I guess it doesn't matter to dig into that
• Also changed a bigint conversion to be cleaner

That one was a lot to debug

Up to block 460 now though :)

Reference issue to close (if applicable)

Closes

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