freeze build timestamp via an environment variable

next.config.js

@@ -23,6 +23,9 @@ module.exports = (phase, { defaultConfig }) => {
     trailingSlash: true,
     productionBrowserSourceMaps: true,
     env: {
+      // freeze build timestamp for when serverless pages need a "last updated" date:
+      RELEASE_DATE: new Date().toISOString(),
+      // check if we're running locally via `next dev`:
       IS_DEV_SERVER: phase === PHASE_DEVELOPMENT_SERVER,
     },
     images: {