pvf: We don't add an execution request when retrying preparation

[mrcnski]

ISSUE

Overview

On repeated execution requests where the preparation previously failed, we retry the preparation under certain conditions:

https://github.com/paritytech/polkadot/blob/30005e6b6e/node/core/pvf/src/host.rs#L527

However, I noticed that we don't do anything with result_tx in this case. I think we are missing a line:

// Add an execution request that will wait to run after this prepare job has finished.
awaiting_prepare.add(artifact_id, execution_timeout, params, result_tx);

I can't remember if there was a reason I left this out, but it seems like a mistake. cc @slumber

[slumber]

Yes, the sender gets dropped. Please also extend a test to ensure that it doesn't happen (try_recv doesn't return Err(Canceled))