Add fail-fast: false to self-hosted test matrix

The self-hosted job matrix was using the default fail-fast: true,
causing one Phoenix failure to cancel all Frontier jobs.

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

Author: sbryngelson

.github/workflows/test.yml

@@ -169,6 +169,7 @@ jobs:
     continue-on-error: false
     timeout-minutes: 480
     strategy:
+      fail-fast: false
       matrix:
         include:
           # Phoenix (GT) — build+test combined in SLURM job