chore: rename slack channel for Fleet's nightly build

[mdelapenya]

What does this PR do?

It uses #elastic-agent instead of #fleet Slack channel for sending notifications.

Why is it important?

New one is more accurate and it narrows down to the right people.

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 for the CLI, 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)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #899 opened

• Start Time: 2021-03-16T09:44:58.685+0000

• Duration: 23 min 6 sec

• Commit: 4159c36

Test stats 🧪

Test	Results
Failed	0
Passed	109
Skipped	0
Total	109

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Test stats 🧪

Test	Results
Failed	0
Passed	109
Skipped	0
Total	109

[mdelapenya]

@EricDavisX from now on, the notifications will be sent to the right channel