devenv up broken with flakes

**Describe the bug**
devenv up not working with flakes

**To reproduce**
use example flake in the guides and run devenv up
https://devenv.sh/guides/using-with-flakes/#modifying-your-flakenix-file

gives the error
```
error: flake does not provide attribute 'packages.x86_64-linux.devenv-up', 'legacyPackages.x86_64-linux.devenv-up' or 'devenv-up'
```

**Version**
devenv: 0.6.3
