luci-base: explicitly invoke busybox applet for password change

Ensure to invoke the Busybox `passwd` applet to change the system password
in a non-interactive manner. Non-Busybox variants may not take the new
password input from stdin or use password hashes which are not supported
by musl's `crypt()` implementation by default.

Fixes: #5629
Signed-off-by: Jo-Philipp Wich <jo@mein.io>