envbuilder: fetch upstream changes from repo if local copy is not dirty

Is it possible to disable the /workspace cache of a coder build with envbuilder.
Even after restart work spaces it caches to old commit

```
#1: 📦 Cloning https://bitbucket.org/lorem/repo.git to /workspaces/ipsum..
#1: 👤 Using no authentication!
#1: 📦 The repository already exists! [672.333µs]
```

Start by tracing the repository reuse and /workspace cache handling that produces “The repository already exists!” in the reported log. Check how local changes are detected and define completion as fetching the upstream commit when the existing copy is clean, while preserving local work when it is dirty.

Written by the indexing model from the issue text.