meson: use run_target for generating tags with ctags

In https://github.com/systemd/systemd/pull/6561, `run_target`
was changed to `custom_target`, which inadvertently caused
relative paths to be passed to ctags due to
https://github.com/mesonbuild/meson/issues/3589.
The switch to `run_target` causes absolute paths to be
passed again and makes it easier to jump from file to
file, hopefully delaying the need to exit Vim :-)

meson.build

@@ -2735,9 +2735,8 @@ if git.found()
                 'tags',
                 output : 'tags',
                 command : [env, 'etags', '-o', '@0@/TAGS'.format(meson.current_source_dir())] + all_files)
-        custom_target(
+        run_target(
                 'ctags',
-                output : 'ctags',
                 command : [env, 'ctags', '-o', '@0@/tags'.format(meson.current_source_dir())] + all_files)
 endif