feat(config): return account id from config in offline mode

[JGAntunes]

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Summary

Fixes FRP-1269 - https://linear.app/netlify/issue/FRP-1269/information-request-is-the-account-id-available-for-build-plugins.

By default, when running in offline/buildbot mode netlify/config doesn't issue any requests to the get site endpoint. This means that until now the resulting @netlify/config didn't have anything else besides the siteId under the siteInfo object. This change will make sure we also return the account_id in accordance with the getSite operation - https://open-api.netlify.com/#tag/site/operation/getSite - if @netlify/config has been provided with an account-id through cli param.

The most important outcome of this change is that it will allow plugins to access account_id at execution time in buildbot through the netlifyConfig object:

• build/packages/build/src/plugins/child/run.js

  Line 29 in d5ef407

  netlifyConfig: netlifyConfigCopy,

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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.

[lukasholzer]

LGTM!