fix: missing environment variables

[kuisathaverat]

Motivation/summary

The CI is failing on PRs for branch 7.x, the cause is three environment variables undefined on the release stage.

Checklist

• I have signed the Contributor License Agreement.
• [ ] I have updated CHANGELOG.asciidoc

I have considered changes for:

• [ ] documentation
• [ ] logging (add log lines, choose appropriate log selector, etc.)
• [ ] metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. [Kibana#44001]([Stack Monitoring] Support endpoint counters in APM kibana#44001))
• [ ] automated tests (add tests for the code changes, all unit tests pass locally)
• [ ] telemetry
• [ ] Elasticsearch Service (https://cloud.elastic.co)
• [ ] Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
• [ ] Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

This PR will run the packaging stage without issues.

Related issues

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #4535 opened

• Start Time: 2020-12-15T09:13:22.380+0000

• Duration: 33 min 30 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4393
Skipped	140
Total	4533

Steps errors

Expand to view the steps failures

Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests

Test Sync

• Took 3 min 12 sec . View more details on here
• Description: ./script/jenkins/sync.sh