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I've just cloned the repo, ran `yarn`, and then `yarn build-for-devtools` as explained in the [README](https://github.com/facebook/react/blob/master/packages/react-devtools-extensions/README.md) but when trying to run `yarn build:firefox:dev` it can't find the packages I just built.

![image](https://user-images.githubusercontent.com/7988472/111039185-9d028380-83e1-11eb-8b45-8e459ec5eaa5.png)

I think I'm following all the instructions here.

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'use strict';
const fs = require('fs');
const fse = require('fs-extra');
const {spawnSync} = require('child_process');
const path = require('path');
const tmp = require('tmp');
const sha = (
  spawnSync('git', ['show', '-s', '--format=%h']).stdout + ''
).trim();
const ReactVersion = JSON.parse(fs.readFileSync('packages/react/package.json'))
  .version;
if (process.env.CIRCLE_NODE_TOTAL) {