"video/x-av1" instead of readable codec name in missing plugins

With GStreamer master from today (ish), and without an AV1 decoder installed, trying to play a WebM AV1 sample will throw an error where "an video/x-av1 decoder" is shown as missing.

gst_missing_plugin_message_get_installer_detail() for the message returns:

gstreamer|1.0|totem|video/x-av1 decoder|decoder-video/x-av1

The description is just video/x-av1 decoder.

The Matroska demuxer that's handling this file has a nice codec name for it:

gst/matroska/matroska-demux.c:    *codec_name = g_strdup_printf ("AOM AV1");