Use internal keys to collect coordinator's fee

[adamPetho]

Fixes: #11406

Only one line changed, but please be very, very, very, very extra careful reviewing and merging this PR.

I want to either test it or write a test for this, but don't know how yet.

[lontivero]

I think a test is not necessary. Anyway, if you want to write one then my recommendation is the following:

• Create a TestWallet (it is a deterministic wallet)
• Set the WabiSabiConfig.CoordinatorExtPubKey = testWallet.GetSegwitAccountExtPubKey()
• Assert that WabiSabiConfig.DeriveCoordinatorScript(index).GetBitcoinAddress(ScriptPubKey.Segwit) == testWallet.CreateNewAddress(internal:true) in a for loop from index = 0 to 10 (or something like that=

[adamPetho]

Thank you @lontivero! I wrote a test just to be 110% sure.