fix(build): Have prepare task signal async completion

PR google#6244 made a change to have the npm run prepare script
(automatically invoked by npm install after package installation)
not bother running the buildJavaScriptAndDeps gulp task when run
from a GitHub action (since our build action will do npm test
straight afterwards, which runs this task already).

Unfortunately, due my misunderstanding of how gulp tasks work
the revised version generated a "Did you forget to signal async
completion?" error from gulp.

Fix this by adding a done parameter and passing the provided
callback to buildJavaScriptAndDeps.  This also allows a
simplification of the the don't-run case by simply calling
done and then returning.

scripts/gulpfiles/build_tasks.js

@@ -295,11 +295,12 @@ var JSCOMP_OFF = [
  * needed, and we don't want to, because a tsc error would prevent
  * other workflows (like lint and format) from completing.
  */
-function prepare() {
+function prepare(done) {
   if (process.env.CI) {
-    return gulp.src('.');  // Do nothing.
+    done();
+    return;
   }
-  return buildJavaScriptAndDeps();
+  return buildJavaScriptAndDeps(done);
 }
 
 const buildJavaScriptAndDeps = gulp.series(buildJavaScript, buildDeps);