bin/gc: Run GC for user-level Guix first•••When there is 0 bytes of disk left, GPG fails to run because it can't
create a lockfile it requires.
This way, we free space before needing GPG.
bin/free-port: Try random ports over sequential ones•••To diminish race-conditions where two separate processes call
free-port(1) and get the same port that is free at that time, but only
one is able to bind to it.