We should wait until after `multi-user.target` is triggered to allow
hardware to finish initializing, such as network devices and USB drives.
This ensures `powertop --auto-tune` sets more tunables to "Good".
Fixes#66820
powertop attempt to load some kernel modules like msr by calling
modprobe. This is the counterpart to
88e43eb39b which has the powertop
executable search PATH for modprobe rather than hardcoding /sbin, and
actually adds the directory containing modprobe to its PATH for the
systemd service.