nixos/systemd: Don't use apply for $PATH

When not using apply, other modules can use $PATH as a list instead of
getting a colon-separated list to each /bin directory.

nixos/doc/manual/release-notes/rl-2009.xml

@@ -777,6 +777,11 @@ CREATE ROLE postgres LOGIN SUPERUSER;
      <link xlink:href="https://github.com/NixOS/nixpkgs/pull/63103">PR #63103</link>.
     </para>
    </listitem>
+   <listitem>
+    <para>
+      <literal>config.systemd.services.${name}.path</literal> now returns a list of paths instead of a colon-separated string.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>