bug#38789: curseradio: executable path failure

[Christopher Howard <christopher.howard@qlfiles.net>]

curseradio package in guix a7a4928 builds fine, but fails during run
time with error:

```
Traceback (most recent call last):
  File "/gnu/store/ac696zknci0rxxr9d0lmd7dr9gyr734l-curseradio-0-
1.1bd4bd0/bin/.curseradio-real", line 11, in <module>
    load_entry_point('curseradio==0.2', 'console_scripts',
'curseradio')()
  File "/gnu/store/ac696zknci0rxxr9d0lmd7dr9gyr734l-curseradio-0-
1.1bd4bd0/lib/python3.7/site-packages/curseradio/__main__.py", line 5,
in main
    curses.wrapper(OPMLBrowser)
  File "/gnu/store/608bvypsh90c58apvd2cgg3m9l2pwjqn-python-
3.7.4/lib/python3.7/curses/__init__.py", line 102, in wrapper
    return func(stdscr, *args, **kwds)
  File "/gnu/store/ac696zknci0rxxr9d0lmd7dr9gyr734l-curseradio-0-
1.1bd4bd0/lib/python3.7/site-packages/curseradio/curseradio.py", line
280, in __init__
    self.interact()
  File "/gnu/store/ac696zknci0rxxr9d0lmd7dr9gyr734l-curseradio-0-
1.1bd4bd0/lib/python3.7/site-packages/curseradio/curseradio.py", line
406, in interact
    stderr=subprocess.DEVNULL, stdin=subprocess.DEVNULL)
  File "/gnu/store/608bvypsh90c58apvd2cgg3m9l2pwjqn-python-
3.7.4/lib/python3.7/subprocess.py", line 775, in __init__
    restore_signals, start_new_session)
  File "/gnu/store/608bvypsh90c58apvd2cgg3m9l2pwjqn-python-
3.7.4/lib/python3.7/subprocess.py", line 1522, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: '/usr/bin/mpv':
'/usr/bin/mpv'
```

This occurs after navigating the menu and attempting to play a radio
station. Obviously the package's mpv executable path was not set
correctly during the build.

-- 
Christopher Howard
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