A0-0000: Use parent hash as seed (#1928)

# Description

Please include a summary of the change and which issue is fixed. Please
also include relevant motivation and context. List any dependencies that
are required for this change.

## Type of change

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- Bug fix (non-breaking change which fixes an issue)
- New feature (non-breaking change which adds functionality)
- Breaking change (fix or feature that would cause existing
functionality to not work as expected)

# Checklist:

<!-- delete when not applicable to your PR -->

- I have added tests
- I have made neccessary updates to the Infrastructure
- I have made corresponding changes to the existing documentation
- I have created new documentation
- I have bumped `spec_version` and `transaction_version`
- I have bumped aleph-client version if relevant

---------

Co-authored-by: Michal Swietek <[email protected]>

pallets/elections/src/impls.rs

@@ -14,7 +14,12 @@ where
     T: Config,
 {
     fn populate_next_era_validators_on_next_era_start(era: EraIndex) {
-        let mut rng = Pcg32::seed_from_u64(era as u64);
+        let parent_hash = frame_system::Pallet::<T>::parent_hash();
+        let mut bytes = [0u8; 8];
+        bytes.clone_from_slice(&parent_hash.as_ref()[..8]);
+        let seed = u64::from_le_bytes(bytes);
+
+        let mut rng = Pcg32::seed_from_u64(seed);
         let elected_committee = BTreeSet::from_iter(T::ValidatorProvider::elected_validators(era));
 
         let mut retain_shuffle_elected = |vals: Vec<T::AccountId>| -> Vec<T::AccountId> {