AppStream metadata is not found if named %{id}.metainfo.xml

I get a warning when building if AppStream metadata is named `%{id}.metainfo.xml` instead of `%{id}.appdata.xml`.

```
[appimage/stderr] WARNING: AppStream upstream metadata is missing, please consider creating it
[appimage/stderr] in usr/share/metainfo/org.naev.naev.appdata.xml
[appimage/stderr] Please see https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps
[appimage/stderr] for more information or use the generator at http://output.jsbin.com/qoqukof.
```

The page that message links to even suggests the use of `%{id}.metainfo.xml`
https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps