Fix swaylock-bg, now a bash script

- reference jq from pkgs
- shellcheck fixes

pkgs/swaylock-bg.nix

@@ -3,18 +3,17 @@ self: with self; ''
   # swaylock
 
   # Make sure we aren't running twice
-  RUNNING=$(ps -A | grep swaylock | wc -l)
-  if [ $RUNNING -ne 0 ]; then
+  RUNNING=$(${procps}/bin/pgrep -c ${swaylock}/bin/swaylock)
+  if [ "$RUNNING" -ne 0 ]; then
     exit 0
   fi
 
-  IMAGE=$XDG_CONFIG_HOME/wallpaper.jpg
+  IMAGE="$XDG_CONFIG_HOME/wallpaper.jpg"
   LOCKARGS=""
 
-  for OUTPUT in `${sway}/bin/swaymsg -t get_outputs | jq -r '.[].name'`
+  for OUTPUT in $(${sway}/bin/swaymsg -t get_outputs | ${jq}/bin/jq -r '.[].name')
   do
       LOCKARGS="''${LOCKARGS} --image ''${OUTPUT}:''${IMAGE}"
-      IMAGES="''${IMAGES} ''${IMAGE}"
   done
   exec ${swaylock}/bin/swaylock $LOCKARGS
 ''