ref(nextjs): Make isBuild rely on NEXT_PHASE environment variable

packages/nextjs/src/utils/isBuild.ts

@@ -2,21 +2,12 @@
  * Decide if the currently running process is part of the build phase or happening at runtime.
  */
 export function isBuild(): boolean {
-  // During build, the main process is invoked by
-  //   `node next build`
-  // and child processes are invoked as
-  //   `node <path>/node_modules/.../jest-worker/processChild.js`.
-  // The former is (obviously) easy to recognize, but the latter could happen at runtime as well. Fortunately, the main
-  // process hits this file before any of the child processes do, so we're able to set an env variable which the child
-  // processes can then check. During runtime, the main process is invoked as
-  //   `node next start`
-  // or
-  //   `node /var/runtime/index.js`,
-  // so we never drop into the `if` in the first place.
-  if (process.argv.includes('build') || process.env.SENTRY_BUILD_PHASE) {
-    process.env.SENTRY_BUILD_PHASE = 'true';
-    return true;
-  }
-
-  return false;
+  // Next.js sets the `NEXT_PHASE` env var depending on what phase/environment we're in.
+  // These phases are constants within the Next.js codebase but sadly they're not exported in a way that we can easily
+  // import them with our rollup setup so we're simply vendoring the relevant constant ourselves.
+  // This constant hasn't changed in the Next.js codebase since next@10:
+  // Most recent: https://github.com/vercel/next.js/blob/406d69d4d9f7d14b9bf497a134f0151914b13964/packages/next/shared/lib/constants.ts#L20
+  // v10: https://github.com/vercel/next.js/blob/118ab7992bc8f7a7e5a7bb996510d9b56ffe4f68/packages/next/next-server/lib/constants.ts#L2
+  const PHASE_PRODUCTION_BUILD = 'phase-production-build';
+  return process.env.NEXT_PHASE === PHASE_PRODUCTION_BUILD;
 }