Return an error if environment variables are not set

The docs show a few different options for environment variable manipulation [here](https://github.com/k0sproject/k0sctl#environment-variable-substitution).

It would be great if we had an option where k0sctl would error if it is not set, similar to the bash `${SOME_VARIABLE?error message goes here}`. I'm using k0sctl in a CI pipeline and utilise environment variables heavily.

Start with the environment variable substitution section in the linked k0sctl documentation and trace the implementation behind those options. Determine how an unset variable is currently handled and identify the relevant tests before defining the error behavior. Done means CI use of an unset variable produces an error, with coverage for the new option and its message.

Written by the indexing model from the issue text.