Unmount `.mnt` earlier
Unmount `.mnt` earlier

This makes certain utilities that recursively traverse the filesystem be
less confused (previously we only unmounted when running command but
not during the build)

Fixes #186

tailhook/vagga,popravich/vagga,tailhook/vagga,tailhook/vagga,tailhook/vagga,popravich/vagga,popravich/vagga,popravich/vagga