Contention over eval cache in CI runs

Multiple jobs in a CI run will fail with errors like:
```
   error (ignored): error: SQLite database '/private/var/lib/github/.cache/nix/eval-cache-v5/3287241d52f3fb7999dd590ad43348cc65b7cf5b041364e2fa52905e28708e22.sqlite' is busy
```

https://github.com/flox/floxenvs/actions/runs/17440598195/job/49522328685#step:6:661

Re-running the job succeeds