perf(config)!: only look up `.{platform}/renovate.json` files if we're running on that platform

_Originally posted by @viceice in https://github.com/renovatebot/renovate/discussions/23386#discussioncomment-6454823_, we should only look up the Platform-speciifc filename if we're running against that Platform, to save unnecessary API/file lookups

However, `renovate-config-validator` should look up any files it can find