Fix JS build on Windows

Summary: Node made a breaking change in a security release for 18/20 where `spawn()` no longer loads `.cmd` files by default. nodejs/node#52554. Execute command in shell.

Differential Revision: D56230965

javascript/just.config.cjs

@@ -218,6 +218,7 @@ function emcmakeGenerateTask() {
 
     return spawn(emcmakeBin, args, {
       stdio: logger.enableVerbose ? 'inherit' : 'ignore',
+      shell: true,
     });
   };
 }