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

Instead of #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 #27664, this
artifact doesn't have any effect on the `hash`, which is used for
generating React version identifier.

DiffTrain build for [78c71bc](78c71bc)

Author: hoxyq

compiled/facebook-www/REVISION

@@ -1 +1 @@
-c47c306a7a23d3c796b148d303764e2832da480c
+78c71bc545bf5c0fdeedc023b69fafe05d988067