[Fleet] Fix root level yaml variables dollar escape

[nchaulet]

Summary

Related to #188377

The way we handle root level yaml variable is done via a regex.replace, that function do not handle $ sign correctly as it's a placehold for matches, (see doc)

That PR fix the usage we do of that function.

How to test

Try to add a custom log integration with the advanced yaml configuration test: $$$$ and verify the policy contains that value.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[obltmachine]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[kpollich]

Great catch! Thanks for the test case as well. 🚀

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 938fc3b

Failed CI Steps

• Jest Integration Tests #2

Metrics [docs]

✅ unchanged

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