update nixpkgs-unstable

- bitcoind 0.20.1 -> 0.21.0
  Manually create a wallet in the backup test because bitcoind
  does not create a default wallet anymore

- disable the failing elementsd build on unstable

test/tests.nix

@@ -187,6 +187,7 @@ let testEnv = rec {
       services.bitcoind.regtest = true;
       systemd.services.bitcoind.postStart = mkAfter ''
         cli=${config.services.bitcoind.cli}/bin/bitcoin-cli
+        $cli createwallet "test"
         address=$($cli getnewaddress)
         $cli generatetoaddress 10 $address
       '';