Disable environment export by default.

[Callek]

Description

This PR fixes #597, by setting exportEnv to a default of false for supply chain security hardening

Checklist

• Added CHANGELOG entry (only for user-facing changes)

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PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

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[Callek]

versioneer-testing seems not to be a GitHub user.

I think this is stale somehow (the check is currently passing) -- versioneer-testing was a mistake of my local git config, which isn't related to this PR in any way.