Sporadic failure on Windows with `FindFirstFileExA`

ninja: error: FindFirstFileExA(note: including file: d:/a/swift-build/swift-build/sourcecache/swift-corelibs-foundation/corefoundation): The filename, directory name, or volume label syntax is incorrect.

I've been unable to isolate the exact scenario that this happens on, but I've seen this happen periodically on GitHub Actions builders recently. It has also happened a couple of times on Azure DevOps builders. The build rules don't seem to have changed, so I suspect that this might be related to a Windows update (the builders do update periodically).