docs: specify virtual env location in .readthedocs.yaml

[eonu]

Description

Specify Poetry virtual environment via environment variable in .readthedocs.yaml

(was breaking Sphinx documentation builds, which now uses Poetry >1.8)

Checklist

• I have added new tests (if necessary).
• I have ensured that tests and coverage are passing on CI.
• I have updated any relevant documentation (if necessary).
• I have used a descriptive pull request title.

[coveralls]

Pull Request Test Coverage Report for Build 67ea09d0-ede2-4adf-8456-ce69a50cfeec

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 94.726%

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Totals
Change from base Build fcace5ec-e3f0-4d58-be6b-e55e9e9e2633:	0.0%
Covered Lines:	2245
Relevant Lines:	2370

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💛 - Coveralls