Bump to current PaymentServer master@HEAD

This gives us the merged version of Ristretto support.

nixos/pkgs/zkapissuer-repo.nix

+let
+  pkgs = import <nixpkgs> {};
+in
+  pkgs.fetchFromGitHub {
+    owner = "PrivateStorageio";
+    repo = "PaymentServer";
+    rev = "94fb418962abee71fa97c09c76e85ccc13cf4c1e";
+    sha256 = "15v71hqhs3rd8c77igbzbi2lbvrb6yyshasq3ijs51w7pwp10dac";
+  }
\ No newline at end of file

nixos/pkgs/zkapissuer.nix

-{ fetchFromGitHub, callPackage }:
+{ callPackage }:
 let
-  paymentServer = fetchFromGitHub {
-    owner = "PrivateStorageio";
-    repo = "PaymentServer";
-    rev = "6fbaac7a14d2a03b74e10a4a82b1147ee1dd7d49";
-    sha256 = "0z8mqmns3fqbjy765830s5q6lhz3lxmslxahjc155jsv5b46gjip";
-  };
+  paymentServer = import ./zkapissuer-repo.nix;
 in
   (callPackage "${paymentServer}/nix" { }).PaymentServer