fix(daemon): replace hardcoded username with $USER in systemd service

Co-authored-by: Le He <AnnatarHe@users.noreply.github.com>

Author: github-actions[bot]

model/sys-desc/shelltime.service

@@ -4,7 +4,7 @@ After=network.target
 
 [Service]
 Type=simple
-ExecStart=/bin/sh -c 'exec $(getent passwd username | cut -d: -f7) -l -c "/usr/local/bin/shelltime-daemon"'
+ExecStart=/bin/sh -c 'exec $(getent passwd $USER | cut -d: -f7) -l -c "/usr/local/bin/shelltime-daemon"'
 Restart=always
 
 # Resource limits