fix(lib): respect hclOutput app config

[jyecusch]

Related issue

Fixes #3875

Description

Only store and read the hclOutput configuration from the manifest (App class), respecting the value provided in the AppConfig, as well as the SYNTH_HCL_OUTPUT environment variable.

Checklist

• I have updated the PR title to match CDKTF's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

[hashicorp-cla-app]

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[jyecusch]

I'm happy to make any changes requested or demo the issue. It may need to be two separate issues, since it would also be nice if the CLI didn't always set the SYNTH_HCL_OUTPUT env var, but running the application directly to get around that is reasonable.

I tried to get the tests running and passing locally following the steps in CONTRIBUTING.md, but I couldn't get them to pass successfully before making changes, so I'm unsure if any tests are broken as a result of this change. I'm happy to fix those too if there are.