[DevTools] chore: read from build/COMMIT_SHA fle as fallback for commit hash (#34915)

This eliminates the gap in a reproducer for the React DevTools browser
extension from the source code that we submit to Firefox extension
stores.

We use the commit hash as part of the Backend version, here:

https://github.com/facebook/react/blob/2cfb221937eac48209d01d5dda5664de473b1953/packages/react-devtools-extensions/utils.js#L26-L38

The problem is that we archive the source code for Mozilla extension
store reviews and there is no git. But since we still download the React
sources from the CI, we could reuse the hash from `build/COMMIT_HASH`
file.