refactor[ci/build]: dont generate sourcemaps for BROWSER_SCRIPT bundles (#27665)

Instead of https://github.com/facebook/react/pull/27664, we can just
exclude `unstable_server-external-runtime.js` from having sourcemaps for
now.

We should consider removing manual copying of this artifact in
52d542ad6d/.github/workflows/commit_artifacts.yml (L136-L138)


As described in https://github.com/facebook/react/pull/27664, this
artifact doesn't have any effect on the `hash`, which is used for
generating React version identifier.

scripts/rollup/build.js

@@ -406,6 +406,10 @@ function getPlugins(
     // UMD builds are rarely used and not worth having sourcemaps.
     const needsSourcemaps =
       needsMinifiedByClosure &&
+      // This will only exclude `unstable_server-external-runtime.js` artifact
+      // To start generating sourcemaps for it, we should stop manually copying it to `facebook-www`
+      // and force `react-dom` to include .map files in npm-package at the root level
+      bundleType !== BROWSER_SCRIPT &&
       !isUMDBundle &&
       !sourcemapPackageExcludes.includes(entry) &&
       !shouldStayReadable;