The `version` field exported by the React package currently corresponds
to the `@next` release for that build. This updates the build script
to output the same version that is used in the package.json file.
It works by doing a find-and-replace of the React version after the
build has completed. This is a bit weird but it saves us from having
to build the `@next` and `@latest` releases separately; they are
identical except for the version numbers.
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