[CI] Update path stack agent steps

[mrodm]

Follows #1959

As it has been changed to use independent Elastic Agents, it is also required to update the folder names used when uploading logs. Now the environment value has the value false.

With this change:

• packages testing with independent Elastic Agents (default now) won't have a suffix in the folder name (e.g. aws).
• packages testing with the Elastic Agent from the stack, it will be added a -stack_agent suffix (e.g. aws-stack_agent).

[elasticmachine]

💚 Build Succeeded

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• Commit: 0229bcf

cc @mrodm