Remove magic number from metadata header trest

internal/artifacts/artifacts_upload_test.go

@@ -71,7 +71,7 @@ func testArtifactsUploadServer(t *testing.T, tempPath string) *httptest.Server {
 			w.WriteHeader(404)
 			return
 		}
-		if !bytes.HasPrefix(metadata[4:], []byte(metadataHeader)) {
+		if !bytes.HasPrefix(metadata, []byte(metadataHeaderPrefix+metadataHeader)) {
 			w.WriteHeader(400)
 			return
 		}

internal/artifacts/metadata.go

@@ -19,6 +19,7 @@ type metadata struct {
 	Comment  string `json:"comment,omitempty"`
 }
 
+const metadataHeaderPrefix = "\x00\x00\x00&" // length of string below, encoded properly
 const metadataHeader = "GitLab Build Artifacts Metadata 0.0.2\n"
 
 func newMetadata(file *zip.File) metadata {