Utilise grunt-cli.

Gruntfile.js

 const loadGruntTasks = require("load-grunt-tasks");
-const { sep } = require("path");
 
 module.exports = grunt => {
   const BUILD_DIR = "build/";
@@ -47,7 +46,7 @@ module.exports = grunt => {
         }
       },
       buildRevealJs: {
-        command: `sh node_modules${sep}.bin${sep}grunt css jshint`,
+        command: "grunt css jshint",
         options: {
           cwd: BUILD_DIR
         }

package.json

@@ -17,6 +17,7 @@
   "license": "MIT",
   "devDependencies": {
     "grunt": "^1.0.4",
+    "grunt-cli": "^1.3.2",
     "grunt-contrib-clean": "^2.0.0",
     "grunt-contrib-copy": "^1.0.0",
     "grunt-exec": "^3.0.0",