bitcoind: init at 29.1

bitcoind 29.x is required for joinmarket.
nixos-25.05 is at 29.0, which is a regression from 29.1.
nixos-unstable is at 30.0, which is incompatible.

Adapted from nixpkgs `96ff46705a694219cefa5155ba52203d9d32e65e`, with
minor modifications to make it compatible with nixos 25.05.

pkgs/default.nix

@@ -25,7 +25,7 @@ let self = {
     mempool-nginx-conf;
   trustedcoin = pkgs.callPackage ./trustedcoin { };
 
-  bitcoind_29 = pkgs.bitcoind;
+  bitcoind_29 = pkgs.callPackage ./bitcoind_29 {};
 
   pyPkgs = import ./python-packages self pkgs.python3;
   inherit (self.pyPkgs)