[2.x] fix: resolve virtual path in -Ypickle-write scalac option (#9011)

earlyOutput is a virtual file reference, so passing it directly via
.toString produces a virtual path that scalac cannot resolve. Use
fileConverter.value.toPath() to convert it to an actual filesystem path.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main/src/main/scala/sbt/Defaults.scala

@@ -1063,7 +1063,11 @@ object Defaults extends BuildCommon with DefExtra {
           )
           if (shouldApplyFlags)
             Def.uncached(
-              Vector("-Ypickle-java", "-Ypickle-write", earlyOutput.value.toString) ++ old
+              Vector(
+                "-Ypickle-java",
+                "-Ypickle-write",
+                fileConverter.value.toPath(earlyOutput.value).toString
+              ) ++ old
             )
           else Def.uncached(old)
         } else Def.uncached(old)