CI: install the external test validators in the test job

ubuntu-autoinstall.test.ts and layout-consistency.test.ts parse the
rendered autoinstall YAML with python3+yaml, and kickstart.test.ts runs
ksvalidator -- none of which the runner image ships. 35 of the 569 tests
failed on exactly that, the same set that fails on a bare workstation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V6HNx6cyr4J4CiMGCy1wkF

.gitea/workflows/ci.yml

@@ -85,6 +85,9 @@ jobs:
       - name: Generate Prisma client (labd typecheck needs it)
         run: pnpm -C src/labd run db:generate
 
+      - name: Install test validators (ubuntu-autoinstall parses YAML via python, kickstart.test runs ksvalidator)
+        run: sudo apt-get update -qq && sudo apt-get install -y -qq python3-yaml pykickstart
+
       - name: Build (needed by completions check)
         run: pnpm build