fix(ui): point the manual systemd unit at the installed CLI path

The service file offered on the site install screen hardcoded
/home/owen/fossorial/cli/bin/pangolin, a developer machine path, so the unit
fails to start on a normal install.

get-cli.sh installs to /usr/local/bin ("Prefer /usr/local/bin for system-wide
installation"), which is also where the bare `pangolin` calls in the surrounding
commands resolve from.

Closes #3768

src/components/newt-install-commands.tsx

@@ -150,7 +150,7 @@ Type=simple
 User=root
 Group=root
 EnvironmentFile=/etc/pangolin/pangolin-site.env
-ExecStart=/home/owen/fossorial/cli/bin/pangolin up site
+ExecStart=/usr/local/bin/pangolin up site
 Restart=always
 RestartSec=2
 UMask=0077