fix(ssr): use dedicated instance per render

[Haroenv]

fixes algolia/instantsearch#5387

We use the same InstantSearch instance for every call of createServerRootMixin, but that means we start multiple times, without destroying, causing memory leaks.

[codesandbox-ci[bot]]

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[Haroenv]

I'll close this for now, as it causes some relevant issues in the tests. They might be fixable, but as the problem doesn't come up often, I'll leave it as-is for now, to avoid disturbing otherwise working implementations