Lock experimental react version in tests (#41100)
x-ref: [slack
thread](https://vercel.slack.com/archives/CGU8HUTUH/p1664579732577999)
x-ref:
https://github.com/vercel/next.js/actions/runs/3162459138/jobs/5149086901
x-ref:
https://github.com/vercel/next.js/actions/runs/3162459138/jobs/5149086956
Also fixes invalid example env file name breaking checkout on Windows
x-ref:
https://dev.azure.com/nextjs/next.js/_build/results?buildId=41449&view=logs&jobId=59111904-15f9-56fe-00af-da61f595b979
## Bug
- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`
