fix(nuxt): only write $nuxt if it's writable

[Haroenv]

In some nuxt 2 cases (https://github.com/algolia/vue-instantsearch/commit/acda29326475bd1b73f12b058a0c02df00b8b239#commitcomment-67430759) this.$nuxt is already defined by the framework, but not a setter, so we can't make it equal to the $nuxt of the component.

Luckily in Nuxt 2, setting $nuxt isn't crucial, and can thus be skipped.

[codesandbox-ci[bot]]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 3613f046aec1cc25166dcbc738610f3445246666:

Sandbox	Source
vue-instantsearch-e-commerce	Configuration