system-config-printer: fix expansion of $(out) in .service file

Allows the configure-printer@.service to find the udev-configure-printer
program (correct ExecStart path).

pkgs/tools/misc/system-config-printer/default.nix

···
 
       # The below line will be unneeded when the next upstream release arrives.
       sed -i -e "s|/usr/bin|$out/bin|" "$out/share/dbus-1/services/org.fedoraproject.Config.Printing.service"
+
+      # Manually expand literal "$(out)", which have failed to expand
+      sed -e "s|ExecStart=\$(out)|ExecStart=$out|" \
+          -i "$out/etc/systemd/system/configure-printer@.service"
     '';
 
   meta = {