[BUILD] fix url joining behavior when downloading precompiled wheels

[Harry-Chen]

Purpose

After #29690 and #29787, the downloading of precompiled wheel might still fail with:

[2025-12-01T16:01:40Z]   urllib.error.HTTPError: HTTP Error 404: Not Found
--
[2025-12-01T16:01:40Z]   Downloading wheel from https://wheels.vllm.ai/nightly/vllm/../../37593deb02423826e9206ff28e77f57a0ff8a0b0/vllm-0.11.2.dev430%2Bg37593deb0-cp38-abi3-manylinux_2_31_x86_64.whl to /tmp/vllm-wheels60bwjpt2/vllm-0.11.2.dev430+g37593deb0-cp38-abi3-manylinux_2_31_x86_64.whl

This is due to urlretrieve not normalizing the levels in url, i.e., nightly/vllm/../../ should be collapsed. Add call to urljoin to solve the problem.

Test Plan

Tested by CI.

Test Result

Shown in CI.

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• 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.
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[gemini-code-assist]

Code Review

This pull request fixes an issue with downloading precompiled wheels by correctly joining URL paths using urljoin. It also corrects the formation of the default CUDA variant name. The changes are logical and address the problem described. I have one suggestion to make the exception handling more robust by catching more specific exceptions, which will prevent masking potential bugs in the script.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Harry-Chen.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork