bake: restore consistent output for metadata

Metadata formatting should not depend on the number
of targets.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

commands/bake.go

@@ -150,14 +150,7 @@ func runBake(dockerCli command.Cli, targets []string, in bakeOptions) (err error
 		return wrapBuildError(err, true)
 	}
 
-	if len(in.metadataFile) > 0 && resp != nil {
-		if len(resp) == 1 {
-			for _, r := range resp {
-				if err := writeMetadataFile(in.metadataFile, decodeExporterResponse(r.ExporterResponse)); err != nil {
-					return err
-				}
-			}
-		} else {
+	if len(in.metadataFile) > 0 {
 		dt := make(map[string]interface{})
 		for t, r := range resp {
 			dt[t] = decodeExporterResponse(r.ExporterResponse)
@@ -166,7 +159,6 @@ func runBake(dockerCli command.Cli, targets []string, in bakeOptions) (err error
 			return err
 		}
 	}
-	}
 
 	return err
 }