Update instructions for running tests against dist

Fixes #2819.

scripts/build/build.js

@@ -131,7 +131,7 @@ shell.cp("README.md", "dist/README.md");
 shell.echo("Done!");
 shell.echo();
 shell.echo("How to test against dist:");
-shell.echo("  1) yarn test --prod");
+shell.echo("  1) NODE_ENV=production yarn test");
 shell.echo();
 shell.echo("How to publish:");
 shell.echo("  1) IMPORTANT!!! Go to dist/");