extraNixConfig breaks package installation

I'm trying to build my devcontainer with the following features;

```json
"features": {
"ghcr.io/devcontainers/features/nix": {
"extraNixConfig": "experimental-features = nix-command flakes",
"packages": "nixd,nixfmt,nixos-rebuild-ng,openssh,git-crypt"
}
},
```

However, no packages are installed. If I remove `extraNixConfig` so that it's just the packages line, then it works correctly and installs the packages.
I have not been able to get this to work, and have had to resort to a fallback of `"postCreateCommand": "echo 'experimental-features = nix-command flakes' | sudo tee -a /etc/nix/nix.conf",` to enable flake support.