do-agent: install .service from upstream

pkgs/servers/monitoring/do-agent/default.nix

@@ -18,6 +18,10 @@ buildGoModule rec {
 
   vendorSha256 = null;
 
+  postInstall = ''
+    install -Dm444 -t $out/lib/systemd/system $src/packaging/etc/systemd/system/do-agent.service
+  '';
+
   meta = with lib; {
     description = "DigitalOcean droplet system metrics agent";
     longDescription = ''
@@ -30,4 +34,4 @@ buildGoModule rec {
     maintainers = with maintainers; [ yvt ];
     platforms = platforms.linux;
   };
-}
+}