nixos/systemd: allow str in systemd.services.<name>.path

Turns out, #75510 was too restrictive.

We also need to allow str here, as some modules set this to
"/run/wrappers" to bring `/run/wrappers/bin` into $PATH of a unit.

nixos/modules/system/boot/systemd-unit-options.nix

@@ -233,7 +233,7 @@ in rec {
 
     path = mkOption {
       default = [];
-      type = with types; listOf package;
+      type = with types; listOf (oneOf [ package str ]);
       apply = ps: "${makeBinPath ps}:${makeSearchPathOutput "bin" "sbin" ps}";
       description = ''
         Packages added to the service's <envar>PATH</envar>