Take bool env var values into account instead of presence

[guilload]

Description

When the Airmail team modifies boolean environment variables, I don't know whether they actually remove them or update the values, so I'd be more comfortable shipping that PR before the next Airmail releases.

How was this PR tested?

• Added unit test

[github-actions]

On SSD:

Average search latency is 0.994x that of the reference (lower is better).
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On GCS:

Average search latency is 1.03x that of the reference (lower is better).
Ref run id: 1969, ref commit: 47caef0
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[trinity-1686a]

i went through all std::env::var calls i could find. These 3 (lambda related) still parse boolean directly: ENABLE_VERBOSE_JSON_LOGS DISABLE_MERGE DISABLE_JANITOR

[guilload]

Well spotted. I will fix those and merge the PR.