fix: install mage on CI

[mdelapenya]

What does this PR do?

It adds mage as a Go package to be used at CI time when building fleet-server's dependencies (using Beats build)

Why is it important?

The build for Fleet Server won't happen, as building metricbeat/heartbeat/filebeat won't be possible as the build tool is not installed at CI.

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 run the Unit tests (make unit-test), and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

Related issues

• Relates feat: run E2E tests for Fleet Server #1628

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-10-18T14:11:00.912+0000

• Duration: 37 min 6 sec

• Commit: 0d4626c

Test stats 🧪

Test	Results
Failed	0
Passed	242
Skipped	0
Total	242

💚 Flaky test report

Tests succeeded.

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