Set GODEBUG defaults and fix env override checks

What does this MR do?

- Fix the checkDuplicateKeyFromEnv template to consider
  both local and global scopes.
- Also set GODEBUG to tlsmlkem=0,tlskyber=0. These was required due to
  an internal discussion on
  gitlab-org&17700. Users can still
  make use of `global.extraEnv` to override this behaviour.

Related issues

Relates to gitlab-org/omnibus-gitlab!8479 (merged), omnibus MR doing the same thing.

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• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
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