update build workflow to use image_json input for arch selection

Author: leighmcculloch

.github/workflows/build.yml

@@ -228,7 +228,7 @@ jobs:
     strategy:
       matrix:
         image: ${{ fromJSON(needs.setup.outputs.images) }}
-        arch: ${{ github.event_name == 'workflow_call' && fromJSON('["amd64"]') || fromJSON('["amd64", "arm64"]') }}
+        arch: ${{ inputs.image_json && fromJSON('["amd64"]') || fromJSON('["amd64", "arm64"]') }}
       fail-fast: false
     name: 5 build (quickstart, ${{ matrix.image.tag }}, ${{ matrix.arch }})
     runs-on: ${{ matrix.arch == 'arm64' && 'ubuntu-24.04-arm' || 'ubuntu-latest' }}