ci: revert case-opt to clean: false to preserve SLURM build cache

The Pre-Build (SLURM) step on Phoenix rebuilds case-optimized binaries
using the runner workspace on NFS. With clean: true, all cached build
artifacts are wiped before each run, causing the SLURM job to build from
scratch and get preempted (~9 min) before completing.

The case-opt job uses explicit GPU opts (gpu-opts.sh) so it is not
affected by the lock.yaml contamination issue that motivated clean: true
in the self job.

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

Author: sbryngelson

.github/workflows/test.yml

@@ -330,7 +330,7 @@ jobs:
       - name: Clone
         uses: actions/checkout@v4
         with:
-          clean: true
+          clean: false
 
       - name: Pre-Build (SLURM)
         if:   matrix.cluster == 'phoenix'