If the busybox-static package is installed, the modprobe implementation
used will be the one from busybox, which behaves slightly differently.
Specifically, the busybox implementation does not support `install`
commands from modprobe.d conf files:

https://git.busybox.net/busybox/tree/modutils/modprobe.c?h=1_31_stable#n279

Since mkinitramfs already ensures that /sbin/modprobe is copied into
/sbin for the initrd, it is safe to fully-qualify the modprobe call and
never invoke the busybox version.