Fix spawn error on Windows

Author: fwang

.changeset/orange-lamps-fail.md

@@ -0,0 +1,5 @@
+---
+"open-next": patch
+---
+
+Fix spawn error on Windows

packages/open-next/src/build.ts

@@ -73,6 +73,7 @@ function buildNextjsApp(packager: "npm" | "yarn" | "pnpm") {
   cp.spawnSync(packager, packager === "npm" ? ["run", "build"] : ["build"], {
     stdio: "inherit",
     cwd: appPath,
+    shell: true,
   });
 }