linux-wallpaperengine: migrate to lib.cli.toCommandLineGNU

Replace deprecated lib.cli.toGNUCommandLine with lib.cli.toCommandLineGNU. 
Should support the GNU style of convention.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/services/linux-wallpaperengine.nix

@@ -117,7 +117,7 @@ in
         args = lib.lists.forEach cfg.wallpapers (
           each:
           lib.concatStringsSep " " (
-            lib.cli.toGNUCommandLine { } {
+            lib.cli.toCommandLineGNU { } {
               screen-root = each.monitor;
               inherit (each) scaling fps;
               inherit (each.audio) silent;
@@ -125,9 +125,11 @@ in
               no-audio-processing = !each.audio.processing;
             }
             ++ each.extraOptions
+            ++ [
+              "--bg"
+              each.wallpaperId
+            ]
           )
-          # This has to be the last argument in each group
-          + " --bg ${each.wallpaperId}"
         );
       in
       {

tests/modules/services/linux-wallpaperengine/basic-configuration-expected.service

@@ -2,7 +2,7 @@
 WantedBy=graphical-session.target
 
 [Service]
-ExecStart=@linux-wallpaperengine@/bin/linux-wallpaperengine --assets-dir /some/path/to/assets --clamping border --fps 6 --scaling fit --screen-root HDMI-1 --bg 12345678 --no-audio-processing --noautomute --screen-root DP-1 --silent --scaling fill --fps 12 --bg 87654321
+ExecStart=@linux-wallpaperengine@/bin/linux-wallpaperengine --assets-dir /some/path/to/assets --clamping border --fps=6 --scaling=fit --screen-root=HDMI-1 --bg 12345678 --no-audio-processing --noautomute --screen-root=DP-1 --silent --scaling fill --fps 12 --bg 87654321
 Restart=on-failure
 
 [Unit]

tests/modules/services/linux-wallpaperengine/missing-spaces-expected.service

@@ -2,7 +2,7 @@
 WantedBy=graphical-session.target
 
 [Service]
-ExecStart=@linux-wallpaperengine@/bin/linux-wallpaperengine --screen-root HDMI-A-1 --bg 2902931482
+ExecStart=@linux-wallpaperengine@/bin/linux-wallpaperengine --screen-root=HDMI-A-1 --bg 2902931482
 Restart=on-failure
 
 [Unit]