Bandersnatch hot fix (#7670)

Essentially, this locks `bandersnatch_vrfs` to a specific branch of a
repository I control. This is a temporary workaround to avoid issues
like https://github.com/paritytech/polkadot-sdk/issues/7653 until
https://github.com/paritytech/polkadot-sdk/pull/7669 is ready.

Closes: https://github.com/paritytech/polkadot-sdk/issues/7653 

@drskalman

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Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>