讀取org.freedesktop.systemd1的變數

https://www.freedesktop.org/software/systemd/man/latest/busctl.html 

https://manpages.ubuntu.com/manpages/focal/zh_TW/man1/busctl.1.html

busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager FirmwareTimestampMonotonic