Don't run package-and-test-ee child pipeline if label is missing

From Draft to Ready

• gitlab-org/quality/triage-ops!2945 (merged) should be merged, and was verified to work as expected

Context

Contributes to gitlab-org/quality/engineering-productivity/team#532 (comment 2003469833)

What does this MR do and why?

Don't run package-and-test-ee child pipeline if label is missing

The label in question is is pipeline:run-e2e-omnibus-once.

If this label is not present, we should NOT run the child pipeline.

If the label is present, we should continue the rules for the child pipeline to see if the job should be run in the pipeline.

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