Update Buildkite pipeline for the new JuliaGPU cluster

[maleadt]

The JuliaGPU Buildkite agents have moved to a dedicated cluster, where the single juliagpu queue has been split into per-backend queues. Steps now select agents using queue: "cuda", queue: "rocm" or queue: "oneapi" instead of queue: "juliagpu" combined with a cuda/rocm/intel tag.

This change only affects agent selection; the steps themselves are unchanged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (89e9259) to head (5161257).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #605   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files          52       52           
  Lines        1463     1463           
=======================================
  Hits         1368     1368           
  Misses         95       95           

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