fish-completion: use the correct fallback directory

fish completions should never be installed to share/fish/completions/ as
that directory is reserved exclusively for completions shipped as part
of the fish source code.

Use the same vendor_completions.d/ directory which the default fish
configuration uses.

meson.build

@@ -214,7 +214,7 @@ if get_option('fish-completions')
 	if fish_comp.found()
 		fish_install_dir = fish_comp.get_pkgconfig_variable('completionsdir')
 	else
-		fish_install_dir = datadir + '/fish/completions'
+		fish_install_dir = datadir + '/fish/vendor_completions.d'
 	endif
 
 	install_data(fish_files, install_dir: fish_install_dir)