Fix CI: remove unused yaml import, pin isogenerator action SHA

- test_release_artifacts.py: remove unused yaml import (ruff F401)
- release.yml: pin ublue-os/isogenerator to commit SHA (check-pins)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: SecAI-Hub

.github/workflows/release.yml

@@ -156,7 +156,7 @@ jobs:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Build ISO
-        uses: ublue-os/isogenerator@main
+        uses: ublue-os/isogenerator@427a070b3ba1d24998833c948b745ef8061bb5e2 # main
         id: isogen
         with:
           ARCH: x86_64

tests/test_release_artifacts.py

@@ -8,8 +8,6 @@
 import re
 from pathlib import Path
 
-import yaml
-
 REPO_ROOT = Path(__file__).parent.parent
 RELEASE_YML = REPO_ROOT / ".github" / "workflows" / "release.yml"
 ARTIFACTS_JSON = REPO_ROOT / "docs" / "release-artifacts.json"