fix(gatsby): stale node manifests

[TylerBarnes]

In some cases it seems there are no pending page data writes but there are still node manifests that should be written out.
I added a stale node manifest check to fix it. Always writing them out is problematic since flush can be called multiple times after node manifests are stored and sometimes there aren't pending writes until the second time it's called. When there are pending writes we want to process the node manifests at the same time as page-data.json files because the node manifest ID is added to page-data.json when they're processed at the same time.

[TylerBarnes]

canary published as gatsby@4.24.8-alpha-stale-node-manifests.1