software/fluentd: Fix SR test on Debian 11

On Debian 10 the test was successful because it could use the system
python (usr/bin/python). In Debian 11 the system python isn't available
anymore, therefore it fails. To ensure the Fluentd program can successfully
execute the "python custom_read_bme280.py" command, the script passes
the same python binary to the Fluentd config file with which the test
is called (sys.executable).