build-tails: drop the -l (lazy) option from the umount call (refs: #12009).

If the build tmpfs cannot be unmounted in practice, then the space it uses
remains as occupied cached memory, which fails the next build. So let's try
to *really* unmount it.

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 -l "$mountpoint"
+		sudo umount -f "$mountpoint"
 		sudo rm -rf "$mountpoint"
 	done
 }