rose metadata: fix env-val triggering

This corrects the default behaviour when triggering from an environment variable value.

The documentation states that a trigger based on a value for a variable should evaluate
true (enable) when that value contains an environment variable. The current behaviour is
that the trigger evaluates false, leading to dependent variables being trigger-ignored.

@matthewrmshin, please review.

@mkjpryor, this came from your recent question.
Is it possible to do:

```python
return rose.env.contains_env_var(value)
```