exec: use flock instead of nq

nq is useful when you need (guaranteed) ordered execution and
persistent queues. The disadvantage is that in combination with docker,
it's not trivial to obtain the command output with nq, as there is only
a single program executing, and you need to run fq separately to get
the output.

Because it's useful to get the command output, flock on linux apparently
still releases the lock in order, and we do not need persistence, switch
to flock instead.