Unset an environment variable via the `env_vars`

[teojgo]

We could have for example the following convention:

env_vars['VAR1'] = None

to results in an unset VAR1 emitted command.

[vkarak]

In addition to this, we'd need to be able to update the environment variables and not just override them when doing -S env_vars=....

[vkarak]

Maybe we should consider a general syntax of += and -= to add/remove from any dictionary or list.

[vkarak]

This is related to #3059.