path_exists: do not break on os-specific env vars

In short, I should be able to write this:

```
[envs.example]
    path_exists = [
        [
            "${USERPROFILE}/file.txt",
            "${APPDATA}/other_file.txt",
        ],
        [
            "${HOME}/file.txt",
            "${XDG_CONFIG_HOME}/other_file.txt",
        ]
    ]
```

and not have Hoard crash.