streamdeck-ui: add systemd user service

pkgs/applications/misc/streamdeck-ui/default.nix

···
       '';
     in
       ''
+        mkdir -p $out/lib/systemd/user
+        substitute scripts/streamdeck.service $out/lib/systemd/user/streamdeck.service \
+          --replace '<path to streamdeck>' $out/bin/streamdeck
+
         mkdir -p "$out/etc/udev/rules.d"
         cp ${writeText "70-streamdeck.rules" udevRules} $out/etc/udev/rules.d/70-streamdeck.rules