Update Buildkite pipeline for the new JuliaGPU cluster. (#605)

Switch to the per-backend queues of the new JuliaGPU cluster.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: maleadt

.buildkite/pipeline.yml

@@ -11,8 +11,7 @@ steps:
       - JuliaCI/julia-coverage#v1:
           codecov: true
     agents:
-      queue: "juliagpu"
-      cuda: "*"
+      queue: "cuda"
     if: build.message !~ /\[skip tests\]/
     timeout_in_minutes: 60