chore: Fix cross-platform preinstall script

[lachlancollins]

I've noticed that the cross-platform preinstall script introduced to here (TanStack/query#4775), used across most tanstack projects, doesn't do what it's supposed to.

The intended (and original) behaviour is to only run only-allow on local dev, and skip on CI. The current script logs "Skipping preinstall..." on CI, but then throws a non-zero exit code. This causes the command after || to run, i.e. it runs only-allow on CI only. You can test this doesn't work locally by changing the last part of script to something like npx -y only-allow yarn.

This PR makes the non-zero code throw only when process.env.CI is not 'true', restoring the intended behaviour.

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