nixos/acme: implement postRun using ExecStartPost

In 5532065d06, acme was changed to be
RemainAfterExit=true, but `postRun` commands are implemented as
`ExecStopPost`. Systemd now considers the service to be still running
after simp_le is finished, so won't run these commands (e.g. to reload
certificates in a webserver). Change `postRun` to use `ExecStartPost` to
ensure the commands are run in a timely manner.

nixos/modules/security/acme.nix

@@ -241,9 +241,9 @@ in
                     StateDirectoryMode = rights;
                     WorkingDirectory = "/var/lib/${lpath}";
                     ExecStart = "${pkgs.simp_le}/bin/simp_le ${escapeShellArgs cmdline}";
-                    ExecStopPost =
+                    ExecStartPost =
                       let
-                        script = pkgs.writeScript "acme-post-stop" ''
+                        script = pkgs.writeScript "acme-post-start" ''
                           #!${pkgs.runtimeShell} -e
                           ${data.postRun}
                         '';