[Pools] fix derivation of pool account (#4999)
closes https://github.com/paritytech-secops/srlabs_findings/issues/408.
This fixes how ProxyDelegator accounts are derived but may cause issues
in Westend since it would use the old derivative accounts. Does not
affect Polkadot/Kusama as this pallet is not deployed to them yet.
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Co-authored-by: Gonçalo Pestana <[email protected]>