chore: rename variable for e2e tests

[mdelapenya]

What does this PR do?

It renames the variable used to trigger the E2E tests for the packages in Beats PRs.

Why is it important?

We renamed the variable to represent a wider scope.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

Related issues

• Needs chore: rename ELASTIC_AGENT_USE_CI_SNAPSHOTS e2e-testing#566 to be merged
• Needs Cherry-pick #22076 to 7.x: Cherry-pick to 7.10: packaging backports #23142 to be merged

Backports

• 7.x
• 7.10

[elasticmachine]

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Build stats

• Build Cause: Pull request #23382 opened

• Start Time: 2021-01-07T07:41:49.647+0000

• Duration: 11 min 42 sec

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