ci: configure system-level conda settings before miniconda setup

[devin-ai-integration]

This PR adds system-level conda configuration before the miniconda setup:

• Removes any existing conda configuration files that might override settings
• Sets system-wide solver to classic explicitly
• Sets system-wide channel priority
• Adds environment variable to enforce classic solver
• Verifies solver configuration before proceeding

This change aims to ensure the classic solver is configured at the system level before any conda operations begin, preventing the libmamba solver from being loaded.

Link to Devin run: https://app.devin.ai/sessions/c6848175de354945a69903fcc437018b

[devin-ai-integration]

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