dracut: fix missing messages for inst.ks=cdrom
    
    fetch-kickstart-disk and anaconda-diskroot are *executed*, not sourced,
    so they don't get the default dracut environment, so "$kickstart" isn't
    set, so "str_starts $kickstart cdrom" is always false, so the user never
    gets any of the "Please insert CDROM containing '$path'..." messages.
    
    Set "$kickstart" from the boot args, and then this all works as
    expected.
    
    Related: rhbz#1168902