Fix relative file paths in coveragerc

[rzabarazesh]

Purpose

This is to fix coverage results not being merged correctly due to tests having differing working_dirs.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[yeqcharlotte]

any test plan? this wouldn't impact vllm/vllm/ right?

[rzabarazesh]

any test plan? this wouldn't impact vllm/vllm/ right?

CI is currently broken. Waiting for it to be fixed so I can run a few test builds: #26065 (review)