ci: run on any runner, amd64 or arm64

The builder image is now multi-arch (linux/amd64 + linux/arm64). Both
variants were tested by running this repo's own CI build steps in each
before being pushed as :latest. Jobs therefore no longer need an amd64
host: runs-on moves to ubuntu-latest, which the amd64 runner (draco-x86)
and the ARM runner (oracle-a1) both carry, so jobs go to whichever is
free instead of queueing behind long jobs on draco.

The Dockerfile header now gives the buildx command for the multi-arch
image, so a rebuild does not silently go back to amd64-only.

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/jray-builder:latest