Revert "ci: Switch to arm preview runners to build base images"

This reverts commit 69221c7.

.github/workflows/backend_base_images.yml

@@ -7,14 +7,7 @@ on:
 
 jobs:
   build-base-images:
-    strategy:
-      matrix:
-        include:
-          - os: ubuntu-24.04
-            platform: amd64
-          - os: ubuntu-24.04-arm
-            platform: arm64
-    runs-on: ${{matrix.os}}
+    runs-on: ubuntu-latest
     permissions:
       packages: write
     steps:
@@ -38,7 +31,7 @@ jobs:
           push: true
           tags: |
             ghcr.io/flathub-infra/backend-builder-base:latest
-          platforms: linux/${{ matrix.platform }}
+          platforms: linux/amd64,linux/arm64
           cache-from: type=gha
           cache-to: type=gha,mode=max
 
@@ -50,6 +43,6 @@ jobs:
           push: true
           tags: |
             ghcr.io/flathub-infra/backend-base:latest
-          platforms: linux/${{ matrix.platform }}
+          platforms: linux/amd64,linux/arm64
           cache-from: type=gha
           cache-to: type=gha,mode=max