Update DD_PKG_GITLAB_URL to prod environment (#44123)

### What does this PR do?

Updates the `DD_PKG_GITLAB_URL` environment variable in `.gitlab-ci.yml` to point to the production artifact gateway URL.

### Motivation

The CI configuration was pointing to the staging artifact gateway (`staging.dog`). This PR updates it to target the production environment (`ddbuild.io`).

**Changed from:**
`https://artifact-gateway.us1.staging.dog/internal/artifact-gateway/api/v4`

**To:**
`https://artifact-gateway.us1.ddbuild.io/internal/artifact-gateway/api/v4`

### Describe how you validated your changes

- [x] Verify CI pipelines can successfully access artifacts from the prod gateway
  - [Successful trigger](https://app.datadoghq.com/logs?query=host%3Ai-0eb365dbd6a3c6bef%20service%3Aartifact-gateway%20container_id%3A8ddbd060e30fcf3689883890db5c5328f12035c5c7b996e3e41f6c94ca8c1e14%20filename%3A0.log&agg_m=count&agg_m_source=base&agg_t=count&clustering_pattern_field_path=message&cols=host%2Cservice&context_event=AZsTbfrdAAAeokimvWtcGAAJ&event=&fromUser=true&messageDisplay=inline&refresh_mode=sliding&storage=hot&stream_sort=time%2Cdesc&to_event=AwAAAZsTcdyWtLUBJAAAABhBWnNUY2Q3NEFBRE9TMHNIR1FYb1VnQUQAAAAkZjE5YjEzNzEtZjcwYi00Y2E5LWFiYjQtYTYzMDA0YmQyOGEzAADaCw&viz=&from_ts=1765557232582&to_ts=1765557787799&live=false)

### Additional Notes

This change affects all CI jobs that use the `DD_PKG_GITLAB_URL` variable for artifact downloads.

Co-authored-by: spencer.gilbert <[email protected]>

Author: spencergilbert

.gitlab-ci.yml

@@ -250,7 +250,7 @@ variables:
   DDA_FEATURE_FLAGS_CI_VAULT_PATH: k8s/gitlab-runner-datadog-agent/datadog-agent/$DDA_CLIENT_TOKEN
 
   DD_PKG_VERSION: "0.6.0-beta.4"
-  DD_PKG_GITLAB_URL: "https://artifact-gateway.us1.staging.dog/internal/artifact-gateway/api/v4"
+  DD_PKG_GITLAB_URL: "https://artifact-gateway.us1.ddbuild.io/internal/artifact-gateway/api/v4"
   PIPELINE_KEY_ALIAS: "alias/ci_datadog-agent_pipeline-key"
 
   # Job cloning strategy