lnworker: fix current_feerate_per_kw for regtest

follow-up 45b248fdef

```
121.72 | I | P/lnpeer.Peer.[LNWallet, 034cd7a09f-e1e44142] | FEES HAVE FALLEN
121.72 | I | P/lnpeer.Peer.[LNWallet, 034cd7a09f-e1e44142] | (chan: 502x1x1) current pending feerate 45000. new feerate 250
121.72 | E | W/lnwatcher.LNWalletWatcher.[default_wallet-LNW] | Exception in update_channel_state: Exception('Cannot update_fee: feerate lower than min relay fee. 250 sat/kw. us: True')
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/util.py", line 1148, in wrapper
    return await func(*args, **kwargs)
  File "/home/user/wspace/electrum/electrum/lnwatcher.py", line 434, in update_channel_state
    await self.lnworker.handle_onchain_state(chan)
  File "/home/user/wspace/electrum/electrum/lnworker.py", line 1216, in handle_onchain_state
    peer.maybe_update_fee(chan)
  File "/home/user/wspace/electrum/electrum/lnpeer.py", line 2221, in maybe_update_fee
    chan.update_fee(feerate_per_kw, True)
  File "/home/user/wspace/electrum/electrum/lnchannel.py", line 1490, in update_fee
    raise Exception(f"Cannot update_fee: feerate lower than min relay fee. {feerate} sat/kw. us: {from_us}")
Exception: Cannot update_fee: feerate lower than min relay fee. 250 sat/kw. us: True
```

electrum/lnworker.py

@@ -2912,10 +2912,11 @@ class LNWallet(LNWorker):
         from .simple_config import FEE_LN_ETA_TARGET, FEERATE_FALLBACK_STATIC_FEE
         from .simple_config import FEERATE_PER_KW_MIN_RELAY_LIGHTNING
         if constants.net is constants.BitcoinRegtest:
-            return self.network.config.FEE_EST_STATIC_FEERATE // 4
-        feerate_per_kvbyte = self.network.config.eta_target_to_fee(FEE_LN_ETA_TARGET)
-        if feerate_per_kvbyte is None:
-            feerate_per_kvbyte = FEERATE_FALLBACK_STATIC_FEE
+            feerate_per_kvbyte = self.network.config.FEE_EST_STATIC_FEERATE
+        else:
+            feerate_per_kvbyte = self.network.config.eta_target_to_fee(FEE_LN_ETA_TARGET)
+            if feerate_per_kvbyte is None:
+                feerate_per_kvbyte = FEERATE_FALLBACK_STATIC_FEE
         return max(FEERATE_PER_KW_MIN_RELAY_LIGHTNING, feerate_per_kvbyte // 4)
 
     def create_channel_backup(self, channel_id: bytes):