ci: increase build timeout to 30m for heavy challenges

EV-12 bot service downloads Chromium via Puppeteer during npm install,
which exceeds the 20-minute limit on CI runners.

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

Author: 0ca

.github/workflows/build-challenges.yml

@@ -59,7 +59,7 @@ jobs:
     name: "Build ${{ matrix.challenge.name }}"
     needs: discover
     runs-on: ubuntu-latest
-    timeout-minutes: 20
+    timeout-minutes: 30
     strategy:
       # Don't cancel other builds if one fails
       fail-fast: false