sysupdate: Implement acquire and install steps for transfers

sysupdate: Implement acquire and install steps for transfers

Instead of using a random temporary path for file transfers, use a
predictable one which indicates whether the transfer is partially
complete or pending installation. Similarly for partitions.

This is another step towards being able to split the ‘update’ step into
‘acquire’ and ‘install’.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://github.com/systemd/systemd/issues/34814