test: monkeypatch some env vars in Predefined Pipelines tests

[anakin87]

Related Issues

Some tests in a PR by an external contributor are failing: https://github.com/deepset-ai/haystack/actions/runs/8175759835/job/22354455607

They (apparently) would need the OpenAI API key to pass, but in external PRs we don't pass the API key.

(Strangely, tests are only failing on Windows.
I can get the same failures locally on Ubuntu if I unset the environment variable. So maybe in Windows, the env var is set but has 0 length)

Proposed Changes:

These specific tests just need the env var to be set, but they don't need to call OpenAI,
so I am monkeypatching the env var.

How did you test it?

Local tests, CI

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 8176786282

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.043%

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Totals
Change from base Build 8175680402:	0.0%
Covered Lines:	5290
Relevant Lines:	5875

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💛 - Coveralls