[engine] support per target env vars setup.

We often need to set, or clear, environment variables for invoking target actions. Add an engine feature to be able to set those per target variables:
- Add special `ENVIRON` per target variable that is composed of a list of _name_ and _value_ pairs. The _value_ can be an empty string to indicate that the _name_ env variable should be unset.
- When the action for a target runs the each _name_ and _value_ in the `ENVIRON` variable is set for the duration of the action invocation.