units: use correct path to refer to plymouth

This doesn't really matter, since in non-/usr-merged systems plymouth
needs to be in /bin and on merged ones it doesn't matter, but it is
still prettier to insert the right path, and avoid /bin on merged
systems, since it's just a compat symlink.

Replaces: #15351

test/test-functions

@@ -879,7 +879,7 @@ install_execs() {
          # also, plymouth is pulled in by rescue.service, but even there the exit code
          # is ignored; as it's not present on some distros, don't fail if it doesn't exist
          dinfo "Attempting to install $i"
-         inst $i || [ "${i%.local}" != "$i" ] || [ "${i%systemd-update-done}" != "$i" ] || [ "/bin/plymouth" == "$i" ]
+         inst $i || [ "${i%.local}" != "$i" ] || [ "${i%systemd-update-done}" != "$i" ] || [ "${i##*/}" == "plymouth" ]
      done
     )
 }