Tentatively fix Service catalog metadata validation failure

### What does this PR do?
Set `parent_environments` to `["staging"]` on the `stable-nightly-build`
and `main-nightly-build` targets of the `datadog-agent-nightly`
conductor.

This does not address why a check advertised as not mandatory fails the
merge queue, which `#sre-core-observability` is investigating.

### Motivation
The `validate-service-metadata` job went red and [puzzled its readers](https://dd.slack.com/archives/C06PBHLD4DQ/p1786712886965919).

It then [blocked the merge](https://dd.slack.com/archives/C0701E5KYSX/p1786719308338409) of #54653.

Asked for advice, its owners confirmed the check is not mandatory, which
seems to hold for the `Merge` button but not for the merge queue, and
[recommended fixing the file meanwhile](https://dd.slack.com/archives/C03BZTDFUKY/p1786722568536699).

The failure code is [`conductor_no_parent_environments`](gitlab.ddbuild.io/DataDog/datadog-agent/-/jobs/1952249035).

### Describe how you validated your changes
Relying on CI: the validator runs as an Atlas workflow, and its offline
counterpart lives in `dd-source`.

### Additional Notes
Being unaware of what the nightly targets feed, I picked "staging" to
mirror the sibling `build_only` target `beta-build-publish-image`, out
of `prod`, `staging`, `gov` and `dr`, so **owners should confirm**.

Author: rdesgroppes

service.datadog.yaml

@@ -94,13 +94,15 @@ extensions:
           branch: "7.81.x"
           schedule: "30 2 * * *"
           build_only: true
+          parent_environments: ["staging"]
           options:
             disable_bia: true
             disable_failure_notifications: true
         - name: "main-nightly-build"
           branch: "main"
           schedule: "0 3 * * 6,0"
           build_only: true
+          parent_environments: ["staging"]
           options:
             disable_bia: true
             disable_failure_notifications: true