ci: disable provenance in metadata to avoid size limits

Docker Buildx 0.31.0 includes full provenance attestations in the
metadata output for multi-platform builds, resulting in 444KB+ JSON
that exceeds bash argument list limits.

Setting BUILDX_METADATA_PROVENANCE=disabled excludes provenance from
the metadata file while keeping attestations attached to images in
the registry.

Related: cloudnative-pg/cloudnative-pg#9826
Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>

Author: mnencia

.github/workflows/bake_targets.yml

@@ -50,6 +50,7 @@ jobs:
         uses: docker/bake-action@5be5f02ff8819ecd3092ea6b2e6261c31774f2b4 # v6
         id: build
         env:
+          BUILDX_METADATA_PROVENANCE: disabled
           environment: testing
           registry: ghcr.io/${{ github.repository_owner }}
           revision: ${{ github.sha }}