relay-chain-consensus: set a fork_choice (#2462)

After #2001 the `fork_choice` strategy may remain uninitialized in the
block import pipeline which uses relay-chain verifier.
Refer to
#2430 (comment)
for some further discussion.

cumulus/client/consensus/relay-chain/src/import_queue.rs

@@ -55,6 +55,10 @@ where
 		&mut self,
 		mut block_params: BlockImportParams<Block>,
 	) -> Result<BlockImportParams<Block>, String> {
+		block_params.fork_choice = Some(sc_consensus::ForkChoiceStrategy::Custom(
+			block_params.origin == sp_consensus::BlockOrigin::NetworkInitialSync,
+		));
+
 		// Skip checks that include execution, if being told so, or when importing only state.
 		//
 		// This is done for example when gap syncing and it is expected that the block after the gap
@@ -100,7 +104,6 @@ where
 		}
 
 		block_params.post_hash = Some(block_params.header.hash());
-
 		Ok(block_params)
 	}
 }