Use lib.warn by default for logging Rollup warnings

This causes the text to be displayed in a more obvious warning color.

lib/rollup.js

@@ -6,7 +6,7 @@ import * as rollup from 'rollup';
 
 /** @param {import('rollup').RollupLog} warning */
 function logRollupWarning(warning) {
-  log.info(`Rollup warning: ${warning} (${warning.url})`);
+  log.warn(`Rollup warning: ${warning} (${warning.url})`);
 }
 
 /** @param {string} path */