fix(gatsby): wait for app-data.json file writing

[MichaelDeBoey]

Follow-up of #22076

[blainekasten]

I need to manually pull this down and test. This seems like the intended behavior given that we await it, but I want to verify nothing breaks. ❤️

[MichaelDeBoey]

@blainekasten We're awaiting it in build

https://github.com/kawamataryo/gatsby/blob/22ead1932af0d5e2a58e3e524fe0bcd2c1ae63ec/packages/gatsby/src/commands/build.js#L115

[pieh]

I'll do some changes to avoid changes that are subjective (they don't change anything in practice, because it's still handled by babel and executed in Node.js context, so this really just change syntax and I would argue that fs.outputJson is easier for grepping etc than outputJson) and only add fixed return type (void -> Promise)