CI: run on dedicated 'paperscan' runner label (x86_64 LXC)

Both the ARM64 global runner and the new LXC runner had the ubuntu-latest
label, so Gitea could dispatch the Android build to the incompatible ARM
box. Use a unique 'paperscan' label so only the x86_64 LXC runner matches.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -9,7 +9,9 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # Dedicated x86_64 runner (LXC op Proxmox) — de globale ARM64-runners hebben deze
+    # label niet, dus zij pikken deze job niet op.
+    runs-on: paperscan
     # Image met Android SDK + JDK 17 + Gradle voorgeïnstalleerd.
     container:
       image: mingc/android-build-box:latest