only capture stdout not stdout+stderr when capture_output=True

[Adam-D-Lewis]

I accidentally was trying to parse stdout + stderr as json instead of just stdout when capture_output=True. This PR fixes that.

Reference Issues or PRs

Fixes failing deployment tests

• e.g. https://github.com/nebari-dev/nebari/actions/runs/10658945768

I validated the idea by re-running one of them and it worked - https://github.com/nebari-dev/nebari/actions/runs/10689796315
Azure now seems to be failing for an unrelated reason - https://github.com/nebari-dev/nebari/actions/runs/10691605764
GCP redeployment - https://github.com/nebari-dev/nebari/actions/runs/10691927249

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

How to test this PR?

Do a local deployment with the "TF_LOG" env var set to "TRACE"

Any other comments?

[viniciusdc]

LGTM, would like to wait for the local tests before merging