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>

Dockerfile.builder

@@ -1,7 +1,10 @@
 # SRFPlay Builder Image
 # Pre-built image with .NET SDK and JPRM for building Jellyfin plugins
-# Build: docker build -f Dockerfile.builder -t gitea.tourolle.paris/dtourolle/srfplay-builder:latest .
-# Push:  docker push gitea.tourolle.paris/dtourolle/srfplay-builder:latest
+# Multi-arch (amd64 + arm64) so runners on ARM hosts such as Oracle Cloud's free
+# Ampere VMs can use it too; the plugin DLLs it produces are identical either way.
+# Build + push (needs a docker-container buildx builder and arm64 binfmt/QEMU):
+#   docker buildx build -f Dockerfile.builder --platform linux/amd64,linux/arm64 \
+#       -t gitea.tourolle.paris/dtourolle/srfplay-builder:latest --push .
 
 FROM mcr.microsoft.com/dotnet/sdk:10.0