Fix awk invocation in the "verify" system test

Fix awk invocation in the "verify" system test

Appending output of a command to the same file as the one that command
is reading from is a dangerous practice.  It seems to have accidentally
worked with all the awk implementations we have tested against so far,
but for BusyBox awk, doing this may result in the input/output file
being written to in an infinite loop.  Prevent this from happening by
redirect awk output to a temporary file and appending its contents to
the original file in a separate shell pipeline.