bootc appears to create a tar file in /tmp for each deleted disk image

Devs:

bootc is somehow creating a backup (?) tar image in TMPDIR for every disk image, possibly for every deleted disk image. Example:

```
-rw-r--r--. 1 jberkus jberkus 2346928640 Sep 8 12:45 0c6a3b5837a2097bd6210f8f6241ad04106761ab8b987699c7ce6291ee731d64-15805.tar
-rw-r--r--. 1 jberkus jberkus 2346928640 Sep 8 12:49 0c6a3b5837a2097bd6210f8f6241ad04106761ab8b987699c7ce6291ee731d64-9556.tar

```

The fingerprints there match a container image for which I made a disk image and deleted it, twice.

The problem with this is these tar files don't ever get cleaned up, which I found out when I ran out of disk space on /tmp.

This is 100% reproduceable, so lemme know what more I can do to test it.