clarify guidance on configuring environment variables for debian installations

[indirectlylit]

migrated from learningequality/kolibri#5556

there was confusion over how env vars should be set when Kolibri is installed to be run as a daemon on kolibri:

@nucleogenesis:

The system used to initialize the Kolibri server systemd will not use environment variables in your .bashrc or other normal files.

@radinamatic:

as all the testing VMs that I use extensively have the KOLIBRI_RUN_MODE env var set up in the .bashrc so they do not skew the Telemetry user stats

@nucleogenesis:

could you post the content of /etc/kolibri/daemon.conf - I have just found and tested that you could also set whatever environment variables you need there and I'd guess that KOLIBRI_RUN_MODE was set there or in a similar file.

Let's determine and document the correct behaviors. cc @jredrejo @benjaoming