fix(ci): exclude publish-benchmark-gh-pages from schedule pipelines

[jbachorik]

What does this PR do?:
Adds a when: never rule for $CI_PIPELINE_SOURCE == "schedule" to the publish-benchmark-gh-pages job.

Motivation:
The reliability (scheduled) pipeline was failing at validation time with errors like:

'publish-benchmark-gh-pages' job needs 'benchmarks-candidate-amd64: [cpu]' job, but it does not exist in the pipeline.

The benchmarks-candidate-amd64/aarch64 jobs have no schedule rule so they never run in scheduled pipelines. The publish-benchmark-gh-pages job, however, had no schedule exclusion — causing GitLab to reject the pipeline because its needs dependencies were absent.

Additional Notes:
The benchmark jobs intentionally don't run on schedule pipelines (they run on trigger/pipeline/web/push sources). This fix aligns publish-benchmark-gh-pages with that constraint.

How to test the change?:
Trigger a scheduled (reliability) pipeline and verify it no longer errors at validation. Trigger a normal pipeline on main and verify publish-benchmark-gh-pages still runs.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.
• JIRA: [JIRA-XXXX]

Unsure? Have a question? Request a review!

[dd-octo-sts]

CI Test Results

Run: #26621524317 | Commit: 2c2156a | Duration: 12m 18s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

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Updated: 2026-05-29 06:33:42 UTC