units: make sure importd has CAP_LINUX_IMMUTABLE flag
    
    Since d8f9686c0f1f276c0a687d9bd69f3adf33f15a95 we use the chattr +i flag
    for marking containers in directories as reead-only. But to do so we
    need the cap for it, hence grant it.
    
    Fixes: #19115
    (cherry picked from commit 86204ae145e38a4557981a92ce91a8ce4318e181)