discovery: fix ireturn on the shared cache reader

CI's golangci-lint flagged ireturn on the generic readCacheFile, which returned its type parameter T. (Locally the nolint directive that silenced it kept getting stripped by golangci-lint run --fix's nolintlint autofix, so it only surfaced in CI.)

Replace it with loadCacheFile(path, *T, newEmpty) error: it fills a destination pointer and returns error, which ireturn allows, while still sharing the read/unmarshal/corrupt-reset logic across both caches.