build: close ssh_channel upon timeout

Merged Jochen Sprickerhof requested to merge jspricke/fdroidserver:destroy_vm into master

The build can get stuck when running into the timeout and paramiko not noticing it. This can be reproduced by setting a small build timeout for an app. I tried setting set_keepalive() but it did not help. This forcefully closes the ssh channel to the VM so the fdroid build will continue. As we destroy the VM in the next step anyhow I think this is fine.