meson: move all .desktop file work into linux conditional

packages/meson.build

-c_pkg = configuration_data()
-c_pkg.set('AEGISUB_COMMAND', 'aegisub')
+conf_pkg = configuration_data()
 
 if host_machine.system() == 'windows'
 elif host_machine.system() == 'darwin'
 else
+    conf_pkg.set('AEGISUB_COMMAND', 'aegisub')
+
     desktop_template = configure_file(input: 'desktop/aegisub.desktop.template.in',
                                       output: 'aegisub.desktop.template',
-                                      configuration: c_pkg)
+                                      configuration: conf_pkg)
 
     i18n = import('i18n')
     i18n.merge_file(input: desktop_template,
@@ -15,7 +16,6 @@ else
                     po_dir: '../po',
                     install: true,
                     install_dir: join_paths(get_option('datadir'), 'applications'))
-endif
 
     aegisub_logos = ['16x16.png', '22x22.png', '24x24.png', '32x32.png', '48x48.png', '64x64.png', 'scalable.svg']
 
@@ -25,3 +25,4 @@ foreach s: aegisub_logos
         install_data('desktop' / dir / 'aegisub.' + ext,
                      install_dir: get_option('datadir') / 'icons' / 'hicolor' / dir / 'apps')
     endforeach
+endif