ci: run on any runner, amd64 or arm64

The builder image is now multi-arch (linux/amd64 + linux/arm64, pushed
as srfplay-builder:latest after building all three plugin zips in each
variant), so jobs no longer need an amd64 host. runs-on moves from
linux/amd64 to ubuntu-latest, which draco-x86, freebox and oracle-a1
all carry, so a job no longer waits behind long jobs on draco while
the ARM runners sit idle.

.gitea/runner/cloud-init.yaml sets up such a runner on an Oracle Cloud
Always Free Ampere VM (Ubuntu 24.04): Docker, act_runner registered
instance-wide with the instance's label set, and a weekly image prune.
The registration token and the optional console password hash are
placeholders here and must be filled in locally before use.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitea/workflows/test.yaml

@@ -17,7 +17,7 @@ on:
 
 jobs:
   test:
-    runs-on: linux/amd64
+    runs-on: ubuntu-latest
     container:
       image: gitea.tourolle.paris/dtourolle/srfplay-builder:latest