fix: preserve cached config while mutating config

[lukasholzer]

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Summary

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

When a build plugin did config mutations, it did not preserve the previous configuration (cachedConfig). As it read it up freshly from the netlify.toml

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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.

[eduardoboucas]

LGTM. Tests are failing, though. Also, do you think it's worth putting this behind a feature flag?

[lukasholzer]

LGTM. Tests are failing, though. Also, do you think it's worth putting this behind a feature flag?

These area is so super well tested, that if all tests pass and I don't have to touch a snapshot I'm pretty confident with the changes that it does not break anything.

The currently solution works fine by passing down the cachedConfig to @netlify/config. It took just so long to test/build as I've forgotten it in one place and therefore the tests where failing. Now I'm pretty confident