Don't use path.join to create globs for gulp.src

Per the gulp documentation[1], globs passed to gulp.src should use '/'
as the path separator regardless of the path separator used on whatever
OS we are running on.

[1] https://gulpjs.com/docs/en/getting-started/explaining-globs#segments-and-separators

scripts/gulpfiles/appengine_tasks.js

@@ -47,8 +47,8 @@ function copyStaticSrc(done) {
  */
 function copyAppengineSrc() {
   const appengineSrc = [
-      path.join(demoStaticTmpDir, 'appengine/**/*'),
-      path.join(demoStaticTmpDir, 'appengine/.gcloudignore'),
+      `${demoStaticTmpDir}/appengine/**/*`,
+      `${demoStaticTmpDir}/appengine/.gcloudignore`,
   ];
   return gulp.src(appengineSrc).pipe(gulp.dest(demoTmpDir));
 }