vendor: re-vendor to add missing file

When running hack/vendor.sh, I noticed this file was added to vendor.
I suspect this should've been part of 0233029d5a,
but the vendor check doesn't appear to be catching this.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/opencontainers/image-spec/specs-go/v1/artifact.go

@@ -0,0 +1,34 @@
+// Copyright 2022 The Linux Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package v1
+
+// Artifact describes an artifact manifest.
+// This structure provides `application/vnd.oci.artifact.manifest.v1+json` mediatype when marshalled to JSON.
+type Artifact struct {
+	// MediaType is the media type of the object this schema refers to.
+	MediaType string `json:"mediaType"`
+
+	// ArtifactType is the IANA media type of the artifact this schema refers to.
+	ArtifactType string `json:"artifactType"`
+
+	// Blobs is a collection of blobs referenced by this manifest.
+	Blobs []Descriptor `json:"blobs,omitempty"`
+
+	// Subject (reference) is an optional link from the artifact to another manifest forming an association between the artifact and the other manifest.
+	Subject *Descriptor `json:"subject,omitempty"`
+
+	// Annotations contains arbitrary metadata for the artifact manifest.
+	Annotations map[string]string `json:"annotations,omitempty"`
+}