Fix removing backup and tempdir after backup and restore

[kke]

Fixes #393

The removal was attempted from CWD, not the tempdir.

Also, the tempdir was not removed in neither backup or restore.

[kke]

time="2022-05-27T11:14:23Z" level=debug msg="[ssh] 127.0.0.1:9022: cleaning up /tmp/tmp.dpijSWOxAw/k0s_backup.tar.gz/tmp/tmp.dpijSWOxAw"
time="2022-05-27T11:14:23Z" level=debug msg="[ssh] 127.0.0.1:9022: executing `rm -f -- /tmp/tmp.dpijSWOxAw/k0s_backup.tar.gz/tmp/tmp.dpijSWOxAw`"
time="2022-05-27T11:14:23Z" level=info msg="* Running clean-up for phase: Prepare hosts"
time="2022-05-27T11:14:23Z" level=fatal msg="PANIC: runtime error: invalid memory address or nil pointer dereference\n"