Use content hash for facebook-www builds (#26331)

Currently, any commit to React causes an internal sync since the Git
commit hash is part of the build. This creates a lot more sync commits
and noise than necessary, see:
https://github.com/facebook/react/commits/builds/facebook-www

This PR changes the version string to be a hash of the target build
files instead. This way we get a new version with any change that
actually impacts the generated files and still have a matching version
across the files.