run minimal test thread-free (#5315)

This increases the end-to-end testing for single-threaded mode which
otherwise might run into problems like
https://github.com/status-im/nimbus-eth2/pull/5303

Makefile

@@ -248,7 +248,8 @@ local-testnet-minimal:
 		--run-geth --dl-geth \
 		-- \
 		--verify-finalization \
-		--discv5:no
+		--discv5:no \
+		--num-threads:1
 
 local-testnet-mainnet:
 	./scripts/launch_local_testnet.sh \