feat(ci-visibility-api-key): Added DD_CIVISIBILITY_API_KEY as override

[CelianR]

What does this PR do?

Motivation

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.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• 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: 2025-11-20 15:41:46

Comparing candidate commit 1ae4a62 in PR branch celian/ci-vis-api-key-alias-acix-1141 with baseline commit 1ed7e54 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 9 metrics, 0 unstable metrics.