Multi-architecture builds are overwriting each other

So we end up with just the arm64 images in ghcr.io repository, since
they are the slowest to build.

Resynchronise the 'platforms' section in the workflow with
upstream. It's not a list of architectures, one per list entry.  It's
a single list entry mentioning all architectures.

.github/workflows/release.yml

@@ -19,8 +19,7 @@ jobs:
           - ./build.sh feature
           - ./build.sh develop
         platform:
-          - linux/amd64
-          - linux/arm64
+          - linux/amd64,linux/arm64
       fail-fast: false
     runs-on: ubuntu-latest
     name: Builds new NetBox Docker Images