Fix side-effect file syncing when using symlink optimization

- Ensure output files are synced even when reading from symlink
- Call findActionResult to get ActionResult for side-effect files
- Maintains performance benefit while ensuring correctness
- Addresses @eed3si9n's concern about tasks generating files on the side

util-cache/src/main/scala/sbt/util/ActionCache.scala

@@ -114,7 +114,13 @@ object ActionCache:
       then
         Exception.nonFatalCatch
           .opt(IO.read(resolvedValuePath.toFile(), StandardCharsets.UTF_8))
-          .map(valueFromStr(_, Some("symlink")))
+          .map: str =>
+            // We still need to sync output files for side effects
+            findActionResult(key, codeContentHash, extraHash, config) match
+              case Right(result) =>
+                store.syncBlobs(result.outputFiles, config.outputDirectory)
+              case Left(_) => // Ignore if we can't find ActionResult
+            valueFromStr(str, Some("symlink"))
       else None
 
     readFromSymlink() match