ci: declare linux-amd64-gpu-l4-latest-1 self-hosted runner label for actionlint

actionlint validates static runner labels against its known set; the new
full-test job uses a literal GPU label (existing GPU jobs dodge this by
building the label from a matrix expression). Declare it so pre-commit's
actionlint hook passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rparolin

.github/actionlint.yaml

@@ -7,3 +7,4 @@
 self-hosted-runner:
   labels:
     - linux-amd64-cpu8
+    - linux-amd64-gpu-l4-latest-1