internal/civisibility: change the default value for DD_CIVISIBILITY_ENABLED to false

[tonyredondo]

What does this PR do?

Change the default value for CI Visibility enabled to be false at least for preview releases. So users require to opt-in using the DD_CIVISIBILITY_ENABLED environment variable.

Motivation

We don't want to enable CI Visibility by default when we are running tests instrumented by orchestrion while we are in preview mode. We will revisit this once we go to GA.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-26 21:49:33

Comparing candidate commit c2bfe58 in PR branch tony/civisibility-disable-by-default with baseline commit 043dcd1 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 58 metrics, 1 unstable metrics.