fix(gatsby-source-wordpress): get missing static file references for cached m…

[arjendejong12]

…edia items

Description

Previously cached media items are collected but nothing is being done with the variable, which leads to missing static files when Gatsby is built.

Related Issues

Fixes #34899

[LekoArts]

You'll want to update the tests, currently it fails on this:

Error: expect(received).toHaveLength(expected)

Expected length: 2
Received length: 4
Received array:  [{"localFile": {"id": 0}}, {"localFile": {"id": 1}}, {"localFile": {"id": 2}}, {"localFile": {"id": 3}}]
    at Object.<anonymous> (/home/circleci/project/packages/gatsby-source-wordpress/__tests__/fetch-referenced-media-items.test.js:135:22)