ci: align build-dev checkout to v6 to match the other jobs

build-dev was the only job still pinning actions/checkout to the v4 hash;
every other job uses v6. Non-functional consistency fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015orsjbLX34FPFGujSK3AQK

Author: TheodoreSpeaks

.github/workflows/ci.yml

@@ -92,7 +92,7 @@ jobs:
             ecr_repo_secret: ECR_PII
     steps:
       - name: Checkout code
-        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
 
       - name: Configure AWS credentials
         uses: aws-actions/configure-aws-credentials@e7f100cf4c008499ea8adda475de1042d6975c7b # v6