Fail if there are undefined variables in the template

As soon as https://github.com/copier-org/copier/pull/2587 is released we should use it.

This should be as simple as:
```yaml
_envops:
undefined: jinja2.StrictUndefined
```

Wait for copier PR #2587 to be released, then inspect the template configuration and identify where the _envops settings belong. Apply the requested undefined-variable behavior and verify that rendering a template with an undefined variable fails as expected.

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