datadog-agent: fix extraIntegrations

The override that builds the custom python for integrations-core was
overriding python, but pythonPackages was still being inherited from a
call to `datadog-integrations-core {}`, causing
service.datadog-agent.extraIntegrations to be ignored.

pkgs/tools/networking/dd-agent/datadog-agent.nix

@@ -3,6 +3,7 @@
 let
   # keep this in sync with github.com/DataDog/agent-payload dependency
   payloadVersion = "4.7.1";
+  python = pythonPackages.python;
 
 in buildGoPackage rec {
   name = "datadog-agent-${version}";
@@ -26,8 +27,6 @@ in buildGoPackage rec {
   goDeps = ./datadog-agent-deps.nix;
   goPackagePath = "github.com/${owner}/${repo}";
 
-  # Explicitly set this here to allow it to be overridden.
-  python = pythonPackages.python;
 
   nativeBuildInputs = [ pkgconfig makeWrapper ];
   buildInputs = [ systemd ];