fix: correct undefined reference to global var with Vitest

[benmccann]

Closes #9162

We should really add tests for this...

[Rich-Harris]

Is there a way we can create this global when Vitest is running, rather than adding code to the bundle for everyone else?

[benmccann]

We could probably change it to the code below, but it seems like it'd only save a byte or two after compression, so I don't know if it's even worth adding that complication?

const global = `${process.env.TEST ? 'globalThis.' : ''}__sveltekit_${version_hash}`;