meson: give the wireplumber binary a variable name

This allows it to be accessed from pipewire when WP is built as a subproject.

Edit: I added another commit to build wp-uninstalled.sh into the builddir. This now allows us to call that script from pipewire without knowing wireplumber's internal repository:

Pipewire's src/daemon/meson.build stanza can now look like this:

elif default_sm == 'wireplumber'
  wireplumber = wp_proj.get_variable('wireplumber')
  wp_bindir = wp_proj.get_variable('wireplumber_bin_dir', pipewire_bindir)                                                            
  conf_config.set('session_manager_path', wp_bindir / wireplumber.name())                                                            
                                                                                                                                                                                                                                                                               
  wp_uninstalled = wp_proj.get_variable('wireplumber_uninstalled')                                                                                                                                                                                                             
  conf_config_uninstalled.set('session_manager_path', wp_uninstalled.full_path())                                                                                                                                                                                              
  conf_config_uninstalled.set('sm_comment', '')                                                                                                                                                                                                                                
endif

The second part relies on !238 (closed) as well, without that we'd have to set the args to wireplumber.name() as well.