If it is a SYSTEM environment variable then you must use setx to change it. See
https://learn.microsoft.com/en-us/wi...-commands/setx. The change is permanent, written to the registry. You will need another setx to reset it
I solve this problems with batch files: one to set both the from-source env var and config directory env var then start calibre-debug, another to set only the config directory env var then start calibre-debug. I don't have a system env var set.