run-postinsts: Fix full execution of scripts at first boot
run-postinsts runs a given set of scripts during the first boot of the
device, when one of these scripts prints something to stdout (isnt
daemonized correctly), since stdout is not available at that time,
the script execution immediately returns with an error (exit_group()),
this error causes the script to terminate all threads within the process,
causing undesired behavior since the script might still had to execute
some other code.
Replace eval built-in with (), since () executes in a subshell,
even if one of the scripts exits, all threads of that process will only
be within that session, this ensures other scripts meant to be run are
still run afterwards.
