[ci][distributed] disable ray dag tests

[youkaichao]

as observed in https://buildkite.com/vllm/ci-aws/builds/6110#019115da-1e57-4e81-8169-872f9b2d100b and https://buildkite.com/vllm/ci-aws/builds/6110#019115da-1e5b-4f18-ba1a-8f0ed3b8fc7d , test starts to error, after executing one ray adag test.

it is possible that #6883 does not work with adag.

will leave for @ruisearch42 and @rkooo567 to investigate.

temporarily disabling ray adag related tests now.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which consists a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of default ones by unblocking the steps in your fast-check build on Buildkite UI.

Once the PR is approved and ready to go, please make sure to run full CI as it is required to merge (or just use auto-merge).

To run full CI, you can do one of these:

• Comment /ready on the PR
• Add ready label to the PR
• Enable auto-merge.

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