[8.x](backport #40238) Go-based changeset definition for MODULE

[mergify]

Proposed commit message

Added MODULE env var definition within mage.
MODULE is set within invoking mage target, if there are changes brought by the PR, instead of calling separate bash-script to evaluate changes and define variable.

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.

Related issues

• Relates: https://github.com/elastic/ingest-dev/issues/3355

Logs

BK builds:

• metricbeat: https://buildkite.com/elastic/beats-metricbeat/builds?branch=oakrizan%3Amage-module-definition
• x-pack/metricbeat: https://buildkite.com/elastic/beats-xpack-metricbeat/builds?branch=oakrizan%3Amage-module-definition
• x-pack/auditbeat: https://buildkite.com/elastic/beats-xpack-auditbeat/builds?branch=oakrizan%3Amage-module-definition

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This is an automatic backport of pull request #40238 done by [Mergify](https://mergify.com).

[botelastic]

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