[CI] docker login for linux only

[v1v]

What does this PR do?

As a consequence of #23854 then it's required the login for Linux workers to interact with the packaging correctly. But it's not required for other platforms such as darwin and windows.

Why is it important?

Docker login in Darwin is not trivial since it requires some docker-machine context, some Darwin workers got no issues regarding the login since it seems the docker-machine context is correct, but some of the CI Darwin workers got some issues.

This should fix the issues of broken builds in the master and 7.x branches regarding those docker login issues.

Workers

The below workers have been disabled until this fix is not merged for master and 7.x branches.

• https://beats-ci.elastic.co/computer/worker-c07ll940dwyl/
• https://beats-ci.elastic.co/computer/worker-c07y20b4jyvy/
• https://beats-ci.elastic.co/computer/worker-c07y20b6jyvy/

[elasticmachine]

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Build stats

• Build Cause: Pull request #24030 opened

• Start Time: 2021-02-13T15:08:28.470+0000

• Duration: 80 min 34 sec

• Commit: 3111609

Test stats 🧪

Test	Results
Failed	0
Passed	45843
Skipped	4762
Total	50605

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Test stats 🧪

Test	Results
Failed	0
Passed	45843
Skipped	4762
Total	50605