display-manager: fix argument handling of sddm

previously session type was not correctly set.

fixes #23264

nixos/modules/services/x11/display-managers/default.nix

@@ -32,8 +32,14 @@ let
     ''
       #! ${pkgs.bash}/bin/bash
 
-      # Handle being called by SDDM.
-      if test "''${1:0:1}" = / ; then eval exec $1 $2 ; fi
+      # SDDM splits "Exec" line in .desktop file by whitespace and pass script path as $1
+      if [[ "$0" = "$1" ]]; then
+        # remove superfluous $1 again
+        shift
+        # join arguments again and evaluate them in a shell context
+        # to interpret shell quoting
+        eval exec "$0" "$@"
+      fi
 
       ${optionalString cfg.displayManager.logToJournal ''
         if [ -z "$_DID_SYSTEMD_CAT" ]; then