fix(next-app-router): do not retrieve headers in csr

[dhayab]

Summary

This PR fixes a side-effect introduced in #6223 , which tries to retrieve a nonce value from headers to apply it to the injected script tag. It unconditionally called Next's headers() in SSR and CSR, which caused the implementation to throw an error at the first CSR search interaction, because headers() are obviously not retrievable from a client environment.

The header() function is now only called in SSR.

[codesandbox-ci]

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 c26e747:

Sandbox	Source
example-instantsearch-getting-started	Configuration
example-react-instantsearch-getting-started	Configuration
example-react-instantsearch-next-app-dir-example	Configuration
example-react-instantsearch-next-routing-example	Configuration
example-vue-instantsearch-getting-started	Configuration