Fix mixed use of `fmt.Errorf` and `xerrors.Errorf`

We should use unified error handling in the project. For simplicity we should just use `fmt.Errorf`.

---

nit: I see there is a mix of `fmt.Errorf` and `xerrors.Errorf`. Should we unify them?

_Originally posted by @mtojek in https://github.com/coder/envbuilder/pull/359#discussion_r1778192905_