dracut/save-initramfs.sh: don't save /tmp
    
    When save-initramfs.sh saves a copy of initramfs, we don't need the
    contents of /tmp. I mean, they're *temporary*. You shouldn't expect them
    to be there later.
    
    This is especially important when we're fetching images (squashfs.img,
    updates.img, product.img, etc.) via http/ftp - they get downloaded into
    /tmp by default, and we really shouldn't be saving extra copies of them
    into an archive in /run.
    
    This should save us ~275MB RAM on PXE-booted systems using http/ftp.
    
    Resolves: rhbz#1148618