more robust: ensure that /run/sshd is present, which is required for ExecStartPre phase;

--- a/src/Pure/Tools/phabricator.scala	Sat Jan 04 20:59:41 2025 +0100
+++ b/src/Pure/Tools/phabricator.scala	Sat Jan 04 21:33:08 2025 +0100
@@ -871,7 +871,7 @@
     Linux.service_install(ssh_name,
 """[Unit]
 Description=OpenBSD Secure Shell server for Isabelle/Phorge
-After=network.target auditd.service isabelle-phabricator-phd.service
+After=network.target auditd.service ssh.service isabelle-phabricator-phd.service
 ConditionPathExists=!/etc/ssh/sshd_not_to_be_run
 
 [Service]