Reapply "Disable buildx provenance"

Removing `provenance: false` works just fine with Artifact Registry, but
it does push 3 tags for each build for reasons unknown to me. Leaving
this setting as-is will be fine.

This reverts commit 917dfa4.

.github/workflows/deploy.yml

@@ -53,6 +53,7 @@ jobs:
         with:
           cache-from: type=gha
           cache-to: type=gha,mode=max
+          provenance: false
           push: true
           tags: |
             gcr.io/${{ env.GCP_PROJECT }}/${{ github.repository }}:${{ steps.vars.outputs.short_sha }}