lnwallet/channel: reuse derived SingleTweak on local force close

When creating the keyring, the tweak is already calculated in the remote
commitment case. We add the calculation also for our own commitment, so
we can use it in all cases without deriving the tweak.

lnwallet/channel.go

@@ -5436,9 +5436,6 @@ func NewLocalForceCloseSummary(chanState *channeldb.OpenChannel, signer input.Si
 	// nil.
 	var commitResolution *CommitOutputResolution
 	if len(delayScript) != 0 {
-		singleTweak := input.SingleTweakBytes(
-			commitPoint, chanState.LocalChanCfg.DelayBasePoint.PubKey,
-		)
 		localBalance := localCommit.LocalBalance
 		commitResolution = &CommitOutputResolution{
 			SelfOutPoint: wire.OutPoint{
@@ -5447,7 +5444,7 @@ func NewLocalForceCloseSummary(chanState *channeldb.OpenChannel, signer input.Si
 			},
 			SelfOutputSignDesc: input.SignDescriptor{
 				KeyDesc:       chanState.LocalChanCfg.DelayBasePoint,
-				SingleTweak:   singleTweak,
+				SingleTweak:   keyRing.LocalCommitKeyTweak,
 				WitnessScript: selfScript,
 				Output: &wire.TxOut{
 					PkScript: delayScript,