perf/bisect_run_script: disable codespeed

When bisecting a performance regression using a config file,
`./bisect_regression --config my_perf.conf` for example, the
config file can contain Codespeed configuration which would
instruct the 'aggregate.perl' script called by the 'run'
script to output results in the Codespeed format and maybe
to try to send this output to a Codespeed server.

This is unfortunate because the 'bisect_run_script' relies
on the regular output from 'aggregate.perl' to mesure
performance, so let's disable Codespeed output and sending
results to a Codespeed server.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>