ci: map runs-on to actual runner labels, trim jobs the runner can't serve

Root cause found for jobs never starting: runs were created all along
(visible in the Actions UI), but every job requested ubuntu-24.04 while
builder-1 only offers ubuntu-latest, linux-build and win-wine - a label
mismatch means jobs queue forever, invisible via the /actions/tasks API
which only lists picked-up jobs.

Mapped build/prepare/gate jobs to ubuntu-latest and the Linux desktop
build to linux-build (electronuserland/builder image, amd64/static only -
the only variant buildable on the single x86 runner). Removed jobs the
runner cannot serve instead of letting them starve the queue: web+desktop
playwright tests (need Docker-in-job and display setup, tracked as
follow-up in issue #2) and the native-Windows desktop build (workflow
assumes signtool/MSVC/PowerShell; the win-wine label awaits a dedicated
wine-based workflow).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/build_desktop_prepare.yaml

@@ -47,7 +47,7 @@ jobs:
     prepare:
         name: Prepare
         environment: ${{ inputs.nightly && 'packages.element.io' || '' }}
-        runs-on: ubuntu-24.04
+        runs-on: ubuntu-latest # builder-1 hat kein ubuntu-24.04-Label
         permissions:
             contents: read
         outputs: