fix: use correct environment file for rendering spa fallback page

[MarkusWendorf]

fixes #10801

The adapter-static fallback page is hardcoded to use production environment variables (this is defined in kit and called from adapter-static). This PR uses the correct environment specified by vite_config.mode.

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[eltigerchino]

Could you move the test to adapter-static/test/apps/spa? I think the fallback page test fits better there since it already has one among the other SPA tests. We can also extend the +error.svelte file to display the environment variable value then navigate to a non-existent route to see the value.

Other than that, it looks really good to me! Thank you!

[eltigerchino]

Thank you!