ci: run the Arch container as amd64 on arm64 runners

.gitea/workflows/ci.yml

@@ -8,7 +8,10 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
-    container: archlinux:latest
+    # The runner may be arm64; Arch Linux only ships x86_64 images, so run them emulated.
+    container:
+      image: archlinux:latest
+      options: --platform linux/amd64
     steps:
       - name: Install dependencies
         run: |