build-system: Retry unmounting if it fails during fmg plugin generation.
Sometimes, the loop device seems to be in use when the unmount is tried
(presumably Nautilus or udisks is looking at it or something). When this
happens, lsof is used to find out which process is occupying the device
(for debugging) and the unmount is retried after sleeping for 5 seconds.
This hasn't been thoroughly tested (since the problem hasn't occured
since I created this fix), but it seems to work at least when the umount
works.