ci: stop Preview docker build from using GHA cache (fixes "failed to reserve cache")

The image build succeeds but exporting to the GHA cache fails with
'failed to reserve cache' under concurrent preview builds against the
10 GiB budget, and buildx treats that as fatal. #3053 (mode=max->min)
did not fix it. Remove cache-from/cache-to type=gha entirely so the
Preview build is decoupled from the cache budget.

.github/workflows/preview.yml

@@ -61,8 +61,6 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=min
           build-args: |
             RELEASE_VERSION=${{ steps.ghd.outputs.describe }}
       - name: Comment on PR