increase GENESIS_OFFSET for local testnet simulation

The 40s allotted for setting up a local testnet can sometime be exceeded
when CI is busy. Bumping it by 20s should reduce that risk.

Example, where the entire slots 0 and 1 were missed:

```
{"lvl":"INF","ts":"2023-11-22 18:33:32.000+00:00","msg":"Scheduling first slot action","topics":"beacnde","startTime":"8s999ms800us826ns","nextSlot":2,"timeToNextSlot":"3s199us174ns"}
```

scripts/launch_local_testnet.sh

@@ -827,7 +827,7 @@ if [[ "$REUSE_EXISTING_DATA_DIR" == "0" ]]; then
     ${REMOTE_URLS}
 fi
 
-GENESIS_OFFSET=40
+GENESIS_OFFSET=60  # See `Scheduling first slot action` > `startTime`
 NOW_UNIX_TIMESTAMP=$(date +%s)
 GENESIS_TIME=$((NOW_UNIX_TIMESTAMP + GENESIS_OFFSET))
 SHANGHAI_FORK_TIME=$((GENESIS_TIME + SECONDS_PER_SLOT * SLOTS_PER_EPOCH * CAPELLA_FORK_EPOCH))