Refine message in `annotate_test_failures` when optional config missing

[mokagio]

I configured Slack reporting in https://github.com/wordpress-mobile/gutenberg-mobile/pull/6134 based on what I saw in WordPress iOS and was surprised by the info message I got. Why did that string work in WP but not on my project? I turns out that WP set the webhook via an env var. Admittedly, It didn't take long to find that out by looking at the code. Still, I decided to update the "Slack skipped" message to something I think would have been clearer for me at the time.

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• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[mokagio]

Thanks @AliSoftware. That was fast! 😄