[Nightly] Correct nightly image build ref

[Potabk]

What this PR does / why we need it?

The underlying tags for nightly image builds have been corrected, and some useless and confusing workflow fields have been removed.

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.14.1
• vLLM main: vllm-project/vllm@dc917cc

[github-actions]

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[gemini-code-assist]

Code Review

This pull request updates the base Docker images for the nightly builds to use the main and main-a3 tags instead of a fixed release version. This change correctly aligns the nightly builds with the latest development code. However, my review highlights a significant risk associated with using floating tags like main. This practice can lead to non-reproducible builds, making it difficult to debug failures caused by the underlying base image. I have recommended switching to immutable tags (e.g., based on commit SHA or date) to enhance the stability and traceability of the build pipeline, which is a high-severity concern for maintaining a robust CI process.