adopt-info not finding desktop files

Version: 2.43.1+18.04

the adopt-info based on appstream isn't working as intended.

the method `_desktop_file_id_to_path` in appstream.py fails to actually resolve the desktop file paths because the working directory is wrong.

I've added

```
        print(desktop_file_path)
        print(os.getcwd())
```

which results in

```
usr/share/applications/org.kde.kblocks.desktop
/home/me/src/git/kblocks
```

this is of course wrong as the working directory should be inside the part's directory `/home/me/src/git/kblocks/parts/kblocks/install/` or thereabout. Or the desktop_file_path relative path would need adjusting to contain the relevant prefix.

This results in `extract` actually not coming back with any desktop files being detected and rendering the automatic desktop file detection from the build state entirely broken.

ADDITIONALLY

The very same problem appears in the `_adopt_keys` method at snapping time, where the working directory is also the actual working directory of snapcraft while the desktop_file_path (which as I understand is basically what `_desktop_file_id_to_path` returned) would also be relative to "root". So it too fails a path check for exactly the same reason.