fix: don't overwrite plugin origin if it exists

[pieh]

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Summary

Part of https://linear.app/netlify/issue/FRP-1218/plugin-netlifyplugin-nextjs-must-not-be-specified-twice-in-the-app

netlify/cli#6781 is related to this and does add default origin to plugins added to default config by framework detection and this change ensures that origin is not being overwritten and instead normalization prefers existing origin over argument provided to addConfigPluginOrigin which later allow validation to pass as it no longer has 2 plugins (one from default config provided by framework detection and one from bitballoon when runtime is set) from same origin (as those are 2 different origins really, not single one)

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[github-actions]

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

[lukasholzer]

Great catch!