Support skip-ci-build label for beatsWhen

[v1v]

What does this PR do?

Support the scenario:

As a contributor I want my PR to be skipped in the CI when I configured it with a github label.

Important

Talking more precisely for Beats this change won't affect the Lint stage since it runs always before the mono-repo stage dynamic generation that happens when using beatsWhen and beatsStages

Why is it important?

Provide fine granularity to configure the builds based on the user expectations too

Related issues

Caused by elastic/beats#20104

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #733 opened]

• Start Time: 2020-09-28T14:03:20.266+0000

• Duration: 8 min 23 sec

Test stats 🧪

Test	Results
Failed	0
Passed	813
Skipped	9
Total	822

Steps errors

Expand to view the steps failures

• Name: Archive JUnit-formatted test results

  • Description: [2020-09-28T14:06:14.181Z] Recording test results
    [2020-09-28T14:06:14.404Z] Failed to read /var/li

  • Duration: 0 min 0 sec

  • Start Time: 2020-09-28T14:06:14.172+0000

  • log