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
https://github.com/facebook/react/blob/52d542ad6d410008c495084f511247f43387055f/.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.