build: move render worker out of app/ into top-level worker/

- oak-render-worker now builds from worker/ (own CMakeLists.txt) as a
  peer of app/; RenderWorkerPool resolves the new build-tree location
- deduplicated the Linux install() rules for the worker
- worker-spawning tests resolve build/worker instead of build/app
- cd.yml: Windows staging copies the worker from its new output path

app/render/renderworkerpool.cpp

@@ -245,6 +245,7 @@ QString worker_program_path()
 	const QStringList candidates = {
 		QDir(app_dir).filePath(file),
 		QDir(app_dir).filePath(QStringLiteral("../app/") + file),
+		QDir(app_dir).filePath(QStringLiteral("../worker/") + file),
 	};
 
 	for (const QString &path : candidates) {