ci: fix duplicate --gpu flag on Frontier GPU test commands

For non-Phoenix GPU jobs, both device_opts and build_opts resolved to
the same --gpu flag. Let build_opts carry it; device_opts is only for
cluster-specific runtime flags like -g (Phoenix GPU IDs).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/common/test.sh

@@ -49,7 +49,8 @@ if [ "$job_device" = "gpu" ]; then
             n_test_threads=$((ngpus * 2))
             ;;
         *)
-            device_opts="$gpu_opts"
+            # Frontier: --gpu flag is already in $build_opts; no extra device opts needed
+            device_opts=""
             n_test_threads=$ngpus
             ;;
     esac