channel backups: fix exception in raised after channel is force closed

Traceback (most recent call last):
  File "/opt/electrum/electrum/util.py", line 1056, in wrapper
    return await func(*args, **kwargs)
  File "/opt/electrum/electrum/lnwatcher.py", line 183, in on_network_update
    await callback()
  File "/opt/electrum/electrum/lnwatcher.py", line 200, in check_onchain_situation
    keep_watching = await self.do_breach_remedy(funding_outpoint, closing_tx, spenders)
  File "/opt/electrum/electrum/lnwatcher.py", line 377, in do_breach_remedy
    sweep_info_dict = chan.sweep_ctx(closing_tx)
  File "/opt/electrum/electrum/lnchannel.py", line 227, in sweep_ctx
    our_sweep_info = self.create_sweeptxs_for_our_ctx(ctx)
  File "/opt/electrum/electrum/lnchannel.py", line 216, in create_sweeptxs_for_our_ctx
    return create_sweeptxs_for_our_ctx(chan=self, ctx=ctx, sweep_address=self.sweep_address)
  File "/opt/electrum/electrum/lnchannel.py", line 321, in sweep_address
    assert self.lnworker.wallet.is_mine(addr)
AssertionError

electrum/lnchannel.py

@@ -491,7 +491,11 @@ class ChannelBackup(AbstractChannel):
         return False
 
     def is_static_remotekey_enabled(self) -> bool:
-        return True
+        # Return False so that self.sweep_address will return self._fallback_sweep_address
+        # Since channel backups do not save the static_remotekey, payment_basepoint in
+        # their local config is not static)
+        return False
+
 
 
 class Channel(AbstractChannel):