Code duplication between `terraform/coder/k8s-ubuntu` and `terraform/coder/k8s-ubuntu-nix`

PR #271 added code duplication. The terraform code is almost identical between `terraform/coder/k8s-ubuntu` and `terraform/coder/k8s-ubuntu-nix`. The only differences are:
* `startup.sh` for `terraform/coder/k8s-ubuntu-nix` contains nix installation + configuration
* `main.tf` and `data.tf` for `terraform/coder/k8s-ubuntu-nix` have one resource more - PVC for `/nix`