install: Run container fetch in host context

Moved from https://github.com/containers/bootc/pull/919#discussion_r1858026043

We already hit in the past a problem where a user had configured their `storage.conf` on the host to use a different location (i.e. not `/var/lib/containers`) and this breaks things right now because our bind mount setup assumes `/var/lib/containers`.

The logistical annoyance here is that we already were doing this correctly before by forking off `skopeo` in the host mount namespace. Except we hit constant problems where people had `podman` but not `skopeo`. And the image proxy API is only in skopeo, not podman.

So fixes:

- Add the image proxy to podman too
- Add Go code here to talk directly to c/image and c/storage (ref https://github.com/containers/bootc/issues/20 )