Fix updating mechanism for Skybian

[jdknives]

Describe the bug
When running the update on the Skybian boards, there is the following error:
Failed to update skychat binary: rename /usr/bin/apps/skychat to /tmp/skywire/apps/skychat.old: rename /usr/bin/apps/skychat /tmp/skywire/apps/skychat.old: invalid cross-device link.

Environment information:
Skybian on an OPi prime.

Possible implementation
I think I need to move the old file to the same location, not in /tmp.