gcr_4: build with ssh-agent

This builds `gcr_4` with ssh-agent, in order to move away from
`gnome-keyring-daemon`, which was [deprecated in 1.46.0](https://gitlab.gnome.org/GNOME/gnome-keyring/-/commit/25c5a1982467802fa12c6852b03c57924553ba73).

Co-authored-by: lilyinstarlight <lily@lily.flowers>

pkgs/development/libraries/gcr/4.nix

@@ -1,4 +1,5 @@
 {
+  pkgs,
   stdenv,
   lib,
   fetchurl,
@@ -25,7 +26,9 @@
   shared-mime-info,
   systemdSupport ? lib.meta.availableOn stdenv.hostPlatform systemd,
 }:
-
+let
+  ini = pkgs.formats.ini { };
+in
 stdenv.mkDerivation (finalAttrs: {
   pname = "gcr";
   version = "4.4.0.1";
@@ -80,11 +83,20 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   mesonFlags = [
-    # We are still using ssh-agent from gnome-keyring.
-    # https://github.com/NixOS/nixpkgs/issues/140824
-    "-Dssh_agent=false"
     "-Dgpg_path=${lib.getBin gnupg}/bin/gpg"
     (lib.mesonEnable "systemd" systemdSupport)
+    "--cross-file=${
+      ini.generate "cross-file.conf" {
+        binaries =
+          {
+            ssh-add = "'${lib.getExe' openssh "ssh-add"}'";
+            ssh-agent = "'${lib.getExe' openssh "ssh-agent"}'";
+          }
+          // lib.optionalAttrs systemdSupport {
+            systemctl = "'${lib.getExe' systemd "systemctl"}'";
+          };
+      }
+    }"
   ];
 
   doCheck = false; # fails 21 out of 603 tests, needs dbus daemon