Hi Guix!

Yesterday I had an interesting conversation on IRC about the behaviour
of multiple `guix package` processes running in parallel. 
Specifically, when two transactions target the same profile (usually
/var/guix/profiles/per-user/$USER/guix-profile) at the same time, one
of them will fail to claim the lock and abort.  0001 makes it so that
the process waits for the lock.  0002 makes it so that packages
specified via -i can be built in parallel.

Regards,
Leo