internal: make container ID logic Linux-only

[nsrip-dd]

What does this PR do?

cgroups are a Linux feature and all of our container ID logic is also
Linux-only. Move all of this behind a linux build tag.

Motivation

Fix the broken Windows build

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.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-19 17:32:50

Comparing candidate commit ecdcd2c in PR branch nick.ripley/fix-entity-id with baseline commit 4ae528b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.

[nsrip-dd]

Closing in favor of #2453