fixup: use the real bindir for systemd unit's bindir

Prior to this commit, the unit contained this line:

     ExecStart=@share/nix-daemon nix-daemon --daemon

Notice how it's trying to execute `share/nix-daemon`, which is unlikely
to exist.

Now with this commit, the path to the daemon binary is properly set:

     ExecStart=@/nix/store/lcbx6d8gzznf3z3c8lsv9jy3j6c67x6r-nix-2.25.0pre20240903_dirty/bin/nix-daemon nix-daemon --daemon

misc/systemd/meson.build

@@ -8,7 +8,7 @@ foreach config : [ 'nix-daemon.socket', 'nix-daemon.service' ]
     configuration : {
       'storedir' : store_dir,
       'localstatedir' : localstatedir,
-      'bindir' : get_option('datadir'),
+      'bindir' : bindir,
     },
   )
 endforeach