chore: use plugin wrapper in nx

Author: ascorbic

demos/nx-next-monorepo-demo/libs/plugin-wrapper/README.md

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+# plugin-wrapper
+
+This empty plugin exists to ensure the demo sites can use `"@netlify/plugin-local-install-core"` to run `npm install`,
+which in turn builds the runtime package. This is needed because `npm install` isn't run in the site if the dependencies
+are unchanged.

demos/nx-next-monorepo-demo/libs/plugin-wrapper/index.js

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+module.exports = require("../../../../packages/runtime/lib/index.js")

demos/nx-next-monorepo-demo/libs/plugin-wrapper/manifest.yml

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+name: 'plugin-wrapper'