fix: route ISO builds to iso-builder runner (ThinkPad only)

VPS runner was sniping jobs and failing instantly (no build env).
Changed runs-on from ubuntu-latest to iso-builder label, which only
the ThinkPad runner has registered.

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

.gitea/workflows/build-iso-dev.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   build-iso:
-    runs-on: ubuntu-latest
+    runs-on: iso-builder
     timeout-minutes: 60
     steps:
       - name: Checkout