Terraform CLI waits for variable input forever if variables are missing

[ansgarm]

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Description

As of #1955 we now invoke Terraform interactively for applying, which causes it to ask for input (e.g. for variables) and to wait basically forever. So the UX to users that don't know Terraform and this output will be confusing.
We have two main options here:

1. Detect when TF asks for input (which we don't support) and crash with an error message that users need to supply variables via TF_VAR_ env variables.
2. Support passing various inputs to the TF CLI (basically extending the "wait for approval" logic for different kinds of questions).

Option 1 is simpler and possibly a good first stepping stone (as detecting those questions is also required for option 2). I'd propose to start with option 1 and then create a follow-up issue to implement option 2 at some point.

References

[DanielMSchmidt]

Same problem with abort, if terraform is killed it sends

Do you want to cancel the remote operation?
  Only 'yes' will be accepted to cancel.

  Enter a value: yes

[github-actions]

I'm going to lock this issue because it has been closed for 30 days. This helps our maintainers find and focus on the active issues. If you've found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.