fix: svelte.dev site scripts filepath generation

[Coronon]

This small change fixes an issue when attempting to run the svelte.dev site locally on a windows machine:

Error: ENOENT: no such file or directory, mkdir '/C:/somepath/svelte/sites/svelte.dev/src/routes/_components/Supporters'

Notice the leading / in the path above, which comes from using .pathname directly instead of fileURLToPath.

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