* allowing to set shell env vars from a key/value map.
* tests

This new map will work alongside the `environment_vars` but it allows using a
datasource value seamlessly. At validation, and because `environment_vars` was
an array of strings containing `KEY=value`, using datsources would not work,
because these values are not known yet and would evaluate to `<unknown>`. With
this, the value and the key can be unknown and will validate correctly for
datasources.