ci: bump prebuild timeout-minutes from 10 to 20

The prebuild job Checkout code step (fetch-depth: 0) was
consistently hitting the 10-minute job-level timeout on GitHub-hosted
Ubuntu runners, cancelling all downstream launcher build steps.

Raising to 20 minutes gives enough headroom for the full-history
fetch on the current repo size while staying well below the build
job 510-minute ceiling.

Author: decentraland-bot

.github/workflows/build-unitycloud.yml

@@ -201,7 +201,7 @@ jobs:
   prebuild:
     name: Prebuild
     runs-on: ubuntu-latest
-    timeout-minutes: 10
+    timeout-minutes: 20
     # Skip when PR has 'perf_test' label (only performance tests should run)
     if: |
       !contains(github.event.pull_request.labels.*.name, 'perf_test') && (