[ROCm] [CI] Fix environment issue

[tjtanaa]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

ROCm vLLM Omni docker image is missing ffmpeg dependencies.

CUDA vLLM Upstream already shipped with ffmpeg dependencies, that is why this vLLM Omni PR #2273 that removed the ffmpeg when removing the librosa dependency does not affect the CUDA CI test.

pyav is just a python wrapper for ffmpeg.

Test Plan

Run the Qwen3 TTS tests on CI to ensure they are passing.

Test Result

https://buildkite.com/vllm/vllm-omni-amd-ci/builds/5120/steps/canvas

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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. Please provide the test scripts & test commands. Please state the reasons if your codes don't require additional test scripts. For test file guidelines, please check the test style doc
• The test results. Please paste the results comparison before and after, or the e2e results.
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model. Please run mkdocs serve to sync the documentation editions to ./docs.
• (Optional) Release notes update. If your change is user-facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

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[gcanlin]

LGTM