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`.

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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_

Search the Go repository for uses of xerrors.Errorf and fmt.Errorf, then review the surrounding error-handling conventions. Replace the mixed usage with the chosen unified approach and run the existing project tests to confirm error behavior remains correct.

Written by the indexing model from the issue text.