Update local testnet script (#4733)

There is an issue with the file `scripts/local_testnet/start_local_testnet.sh` - when we use a non-default `$SPEC-PRESET` in `vars.env` it runs into an error: 
```
executing: ./setup.sh >> /home/ck/.lighthouse/local-testnet/testnet/setup.log
parse error: Invalid numeric literal at line 1, column 7
```
@jimmygchen found the issue and the updated script includes the flag `--spec $SPEC-PRESET`

scripts/local_testnet/start_local_testnet.sh

@@ -103,7 +103,7 @@ echo "executing: ./setup.sh >> $LOG_DIR/setup.log"
 ./setup.sh >> $LOG_DIR/setup.log 2>&1
 
 # Update future hardforks time in the EL genesis file based on the CL genesis time
-GENESIS_TIME=$(lcli pretty-ssz --testnet-dir $TESTNET_DIR BeaconState $TESTNET_DIR/genesis.ssz | jq | grep -Po 'genesis_time": "\K.*\d')
+GENESIS_TIME=$(lcli pretty-ssz --spec $SPEC_PRESET --testnet-dir $TESTNET_DIR BeaconState $TESTNET_DIR/genesis.ssz | jq | grep -Po 'genesis_time": "\K.*\d')
 echo $GENESIS_TIME
 CAPELLA_TIME=$((GENESIS_TIME + (CAPELLA_FORK_EPOCH * 32 * SECONDS_PER_SLOT)))
 echo $CAPELLA_TIME