Build: re-run browserify when generating site (#9275)

(fixes eslint/archive-website#407)

This updates the `npm run gensite` script to run browserify as part of the site generation, rather than using whatever it finds in `build/eslint.js`. This has a few advantages:

* The built file will always be up-to-date (previously, it would reflect the state of the repository whenever `npm test` was last run).
* The gensite script won't fail if it's run on its own and there isn't a `build/` directory.

Makefile.js

@@ -792,6 +792,7 @@ target.gensite = function(prereleaseVersion) {
     // 13. Update demos, but only for non-prereleases
     if (!prereleaseVersion) {
         echo("> Updating the demos (Step 13)");
+        target.browserify();
         cp("-f", "build/eslint.js", `${SITE_DIR}js/app/eslint.js`);
     } else {
         echo("> Skipped updating the demos (Step 13)");