Force cloning or pull of repo on each run of envbuilder

When using envbuilder and restarting my workspace I nearly always want a fresh copy of the repo, but when using a pvc similar to this [example template](https://github.com/coder/coder/blob/main/examples/templates/devcontainer-kubernetes/main.tf) I envbuilder simply finds the existing repo, prints:

```
#1: 📦 Cloning MY_REPO...
#1: 📦 The repository already exists! [5.29944ms]
```

I see no way to force envbuilder to either clone it each time or, preferably, run a git pull to ensure the repo is fresh between rebuilds, but this would make the process more intuitive for our users.