Also update companions of companions before merge

[joao-paulo-parity]

This is yet another "companion build system design problem" to add onto #327.

Showcased by the interaction between paritytech/substrate#10463, paritytech/polkadot#4519 and paritytech/cumulus#870.

Similar to what was done for paritytech/pipeline-scripts#23, in case a Substrate PR has companions on Polkadot and Cumulus, it's not enough to update only Substrate for Cumulus: the Polkadot PR should be updated into the Cumulus PR as well.

Instead of the current:

1. Merge Substrate PR
2. Update lockfile of Polkadot and Cumulus
3. Wait to merge

It should be

1. Merge Substrate PR
2. Update the lockfile of the Polkadot PR
3. Wait for Polkadot PR to be merged
4. Update the lockfile of Cumulus with both Substrate and Polkadot
5. Merge Cumulus PR

[joao-paulo-parity]

This needs #359 before it is released