build-tails: make the umount call verbose (refs: #12009).

… in the hope that it tells us why the filesystem cannot be unmounted,
if this ever happens.

vagrant/provision/assets/build-tails

@@ -30,7 +30,7 @@ cleanup() {
 remove_build_dirs() {
 	for mountpoint in $(old_build_dirs | tac) ; do
 		sudo lsof | grep --fixed-strings "$mountpoint" || true
-		sudo umount -f "$mountpoint"
+		sudo umount -f --verbose "$mountpoint"
 		sudo rm -rf "$mountpoint"
 	done
 }