[ci] generate public CF url in slack notification

[avtolstoy]

Problem

$CF_BUILD_URL generated by CF contains a private URL requiring CF sign-in even though we have public build logs enabled on CF.

Solution

Patch it.

Steps to Test

Take a look at Device OS bots channel for build logs out of this branch/PR - the URL should be public and viewable without sign-in.

Example App

N/A

References

N/A

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)