feat: fetch integrations only when online

[khendrikse]

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Summary

Right now we always fetch integrations, even when the offline option is being used.

build/packages/config/src/main.ts

Line 120 in 08dbe97

const mergedIntegrations = await mergeIntegrations({

This was mentioned by @eduardoboucas in this thread:
https://netlify.slack.com/archives/C043UGRAC3F/p1699613379190979

Let's make sure we don't do that when the offline boolean is true.

Fixes https://linear.app/netlify/issue/CT-272/bug-only-fetch-integrations-if-offline-is-false-in-netlify-build

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[eduardoboucas]

LGTM. It would be cool to cover this with a test.