fix: update model_cost_map_url to use environment variable

[mcowger]

Title

Update model_cost_map_url to use environment variable when available, as decribed in the docs (docs/my-website/docs/proxy/sync_models_github.md)

Relevant issues

Fixes: #15274

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• I have added a screenshot of my new test passing locally
• My PR passes all unit tests on make test-unit
• My PR's scope is as isolated as possible, it only solves 1 specific problem

Type

🐛 Bug Fix

Changes

read the URL from the ENV var, when set.

[vercel]

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[mcowger]

Note: Reviewed all failing tests, and they also fail without the (small) code change in this PR. FYI @ishaan-jaff