fix: leverage the internal config inside the getEnv as well

[lukasholzer]

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Summary

leverage the internal config inside the getEnv as well, the internal config is set through the cached config by the CLI

Needed for: https://linear.app/netlify/issue/CT-738/cli-should-use-framework-detection-and-install-runtime-for-build-and

This is needed in addition to: #5761

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