[fix] update CWD before running postinstall sync

[Conduitry]

Fixes #5769, hopefully. It's really hard to know this sort of thing for sure until we merge and publish.

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[Conduitry]

I'm realizing in now that perhaps I shouldn't have done quite so much cleanup here, because it might still be nice to know the full path to the svelte.config.js file we didn't find. I can add that back in if desired.

[Conduitry]

I went ahead and did it. I convinced myself that that was better without anyone else having to do it. The Missing ... - skipping message now has the full path again.